Got this working and tested with a couple of Memtest86+ 5.01 passes with no issues. BIOS, HwINFO64 and Windows 10 all report 64GB running at 2666 MHz. BIOS and HwINFO64 report Dual InterLeave and Dual-Channel. (I'm still running 1.10.0 BIOS and Windows 10 Pro 1903.)
There are a number of reports of difficulties figuring out compatible RAM (regardless of SODIMM capacity) for the Dell XPS 15 9550 (as well as 9570). The pattern that seems to emerge is that these systems don't like RAM with low CAS Latency. The odds of being compatible seems to increase with at least a certain minimum CAS Latency (CL) timing for a given speed. The following is likely ok with the 9550:
2133 MHz - CL 15 or greater
2400 MHz - CL 17 or greater
2666 MHz - CL 19 or greater
So, for example, 2133 at CL 16 would probably work, but 2400 at CL 16 might not.
I was hesitant to try 64GB due to reported upgrade incompatibilities, but came across someone successfully using a single 32GB SODIMM in the 9550 here. The clincher, though, was an Amazon review on their Australian website where a user got 2x32GB working in the Dell XPS 15 9550 here (scroll down to 2 Australia Customer reviews).
The SODIMMs I purchased and installed are same as users reported above - two Samsung M471A4G43MB1 32GB DDR4 2666MHz, available on Amazon here. (Note Dell's 9550 manual mentions the laptop main battery should be disconnected before changing out RAM.)
The one thing I can't speak to is any change in power usage and battery life, since I primarily use the laptop when connected to A/C.
Now, to (easily) use up all that memory with my software development and emulation/simulation projects...
Some screen images to go with the above post - HWiNFO64 showing RAM timings and BIOS screen showing Dual InterLeave. Note Samsung M471A4G43MB1 is running at CL 19 and 2666 MHz. HWiNFO64 shows memory clocks at half nominal speed (2 x 1333 = 2666). (As an aside, note minimum CAS Latencies I mentioned above are not a hard and fast rule - I've seen exceptions.)
When I put the back cover on after replacing the memory I finger tightened the small Torx T5 screws only until they weren't loose. Pressing the bottom and top of the case together helps see how much farther the screws have to go and avoids using the screw to tighten the cover. The T5 screws seems to screw into aluminum, which is easy to strip.
64GB of RAM seems to be the maximum memory capacity for the Dell XPS 15 9550 for now (unless they come out with 64GB SODIMMs, which may or may not work in the 9550).
Can XPS 9550 take 1.35V dimms ?
From your examples I can see 1.2V dimms used.
However, high voltage ram seem to have better CAS.
So 64Gb means more current. Can we also add more voltage on top of it ?