"I saw mentions of the Seasonic Focus PSU, but it doesn't seem to come with a UK plug." Can't you just use the power cable that came with the XPS 8930 with the Seasonic power supply? The Seasonic power supply voltage range is 100-240 VAC.

I've never been too keen on mixing plugs with hardware they didn't come with. 

According to the Amazon UK site, the Seasonic comes with an EU plug, so that will not work for you. This post by HanoverB mentions other power supplies that will work including the Corsair RMX Series RX850X: https://www.dell.com/community/XPS-Desktops/XPS-8930-GPU-and-CPU-Liquid-Cooler-PSU-Case-Swap-Upgrade/m-p/6137734/highlight/true#M16377

It seems that the RM850X, RM750X and the RM650X all have the same dimensions, 150x86x160 whereas the Seasonic is 140x86x150.

I do not think coil whine can be attributed to how much power margin you have remaining in your power supply. Coil whine is a result of the power regulating process and sometimes the frequency is such that you can hear it. The black screens and freezing could be the result of issues with video drivers or game software. I don't recall any reports of such problems related to Dell updates in this forum.

Hoping its not the GPU. I know this model can have some heating issues, but I supplement the cooling with a USB desk fan running off a power strip, just to add some additional cool air toward the vents when gaming. The crash would trigger during lower intensity gaming more often than graphically intense moments. 

I'm considering going for the RM850x model since the cost difference between it and the 750 isn't huge and they're the same size. I've seen people mention about requiring a cable for an optical drive, but I can't seem to narrow down which one I might need. I have a laptop style DVD drive in my system. 

"I've seen people mention about requiring a cable for an optical drive, but I can't seem to narrow down which one I might need." The thread I linked to earlier had a link to this mini-SATA cable required for optical drive with PSU upgrade: https://www.amazon.com/gp/product/B07FK5G2TF

Unfortunately the same cable doesn't show up on Amazon.co.uk. 

Edit: Another check shows it is, but its unavailable. 

Not sure why you say "Seasonic comes with an EU plug"

Most all Dell units can use various power cords.

So when you are in USA you use NEMA 5-15 to C13 Power Cord 

https://www.dell.com/en-us/work/shop/american-power-conversion-c13-to-nema-5-15p-power-cord-8-ft/apd/a7097192/power-cooling-data-center-infrastructure

in Europe you use SHUKO  CEE7/7 to C13 cord

https://www.dell.com/en-uk/work/shop/c2g-c13-to-cee7-7-european-power-cord-black-2m/apd/a6929301/power-cooling-data-center-infrastructure

and in UK /Ireland

BS 1363   to IEC 60320 C13 cord.

https://www.dell.com/en-uk/shop/c2g-c13-to-bs-1363-uk-irish-power-cord-black-2m/apd/a6927684/power-cooling-data-center-infrastructure

All with the same power supply that auto switches between 110 to 250v.

The issue you need to worry about are the physical dimensions of the power supply.  Some of them are too large to fit into the case.

Thats why I recommend EVGA 700BR as minimum upgrade.

https://www.bestbuy.com/site/evga-700w-atx12v-eps12v-80-plus-bronze-power-supply-dc-dc-technology-black/6346160.p?skuId=6346160

There will also be a cable needed for the optical drive  15 pin to 6 pin mini sata power.

https://www.amazon.com/CRJ-15-Pin-Slimline-Sleeved-Adapter/dp/B07FK5G2TF/

I've just ordered an RM850x as well as that cable for the disc drive (managed to order from U.S. Amazon).

The RM850x dimensions are 150 x 86 x 160 mm (WxHxD). I do hope this will fit? A previous thread that was linked mentioned it would.

As an additional question @Vic384 . I received my RM850x PSU today, though I'm still waiting on the DVD adapter cable.

Given some of the heating issues in the XPS 8930, would my new PSU (which I presume to be slightly larger than the OEM) cause any increased airflow or heating issues?

Should I take the step of getting one of these as well for the CPU? I've seen some recommending this for the 8930, but I don't know if it would fit in my planned new setup.

https://www.scan.co.uk/products/arctic-freezer-11-lp-low-profile-cpu-cooler-for-intel-socket-1150-1151-1155-1156-775-100-watts-max-c

I don't really know much about all of this, but would it fit/replace the blower/be a good idea to do? The fan itself is pretty cheap, and if it makes a beneficiary difference that's worth it. 

@Vic384 

I managed to locate a 120mm fan as often recommended in other threads, but the problem seems to be the mounting bracket that you need, which only appears to be sold on eBay (https://www.ebay.co.uk/itm/Dell-XPS-8910-8920-Alienware-Aurora-R5-Top-Cooling-Case-Fan-7M0F5-X64T5/233144016132?_trkparms=aid%3D888008%26algo%3DDISC.CARDS%26ao%3D2%26asc%3D20131227121020%26meid%3D5231077bda3e4151a8c2f618f99abe33%26pid%3D100009%26rk%3D1%26rkt%3D1%26mehot%3Dag%26sd%3D352340402578%26itm%3D233144016132%26pmt%3D0%26noa%3D1%26pg%3D2047675&_trksid=p2047675.c100009.m1982), and even then, I'd hate to receive a fan that didn't come with that bracket attached, making it pointless.

I hate that this system came with inadequate components to handle the hardware, forcing you to jump through hoops trying to track down parts to correct it all, especially what was paid for it. 

Would the blower CPU cooler be an option as well to look at, the one they apparently fit to the K model processors? 

I would think about replacing the top fan if your XPS 8930 has one of those smaller top fans like that discussed here: https://www.dell.com/community/XPS-Desktops/XPS-8930-SE-top-fan-not-same-as-XPS-8920/td-p/7190716

There is also some discussion in this forum about adding a front fan. See this: https://www.dell.com/community/XPS-Desktops/XPS-8930-GPU-and-CPU-Liquid-Cooler-PSU-Case-Swap-Upgrade/m-p/6138060/highlight/true#M16396

I think those changes would improve cooling more than changing the CPU cooler. The top fan will help air flow in the top half of the case and the bottom front fan will help with air flow in the bottom of the case. The slightly larger power supply should not make any difference although being modular will help because you may not have as many cables to block air flow.

@Moab23 Yes, it appears to only be sold on eBay, but it appears to come with the fan attached to the bracket.

Yes, the CPU heatsink and fan that comes with the K processors is an option but the price is around $82. Here is a link about that heatsink and fan: https://www.dell.com/community/XPS-Desktops/XPS-8930-Special-Edition-heatsink-question/td-p/7445043

@Vic384 

I think I'll call Dell tomorrow and see if I can't get them to cough up a fan and/or bracket first, otherwise I'm going to be spending £36 just for a bracket I should already have, considering the fans are pretty cheap by comparison.

Do you think pursuing something like the Arctic CPU cooler would be more efficient than the stock fan they've fitted to the CPU? I'm going to try everything I can within reason to reduce temperatures if I can, even by a little.