If you're getting 150 Mbps, then your Ethernet connection isn't running at 100 Mbps.  Also, it's never a good idea to use Internet connection speeds to determine the link speed of your network adapter, which is negotiated between the PC and whatever is physically on the other end of the cable.  To see that, go to Control Panel > Network and Sharing Center > Change adapter settings.  Double-click your Ethernet adapter and see whether that shows 100 Mbps or 1 Gbps. I'm betting it's the latter if you're already getting 150 Mbps on your browser, and in that case, your bottleneck would be elsewhere.  Any chance you're also connected via WiFi and Windows is using that connection instead?  And what site/tool have you been using to benchmark your Internet speeds thus far anyway?

(And side note, when talking about compression, the actual speed is always lower, i.e. it would be 500 Mbps actual, but up to 600 Mbps effective when compression is enabled.)

In addition to the above, have you been messing with linespeed and/or duplex settings on your laptop and/or whatever switch/router you're plugging into?  If so, set all of that back to Auto, because sometimes severe performance degradation can occur when you hard-code those settings, even if those are the settings that would have been negotiated anyway, and sometimes even when both sides are hard-coded the same way.  The official recommendation in the Gigabit spec is to use Auto across the board.  Have you tried replacing the cable or at least testing some other device using that same cable immediately after testing your laptop in order to see whether the other device can in fact get higher speeds on that cable at virtually the same time of the same day?

As for the "addition",

- No messing with linespeed and/or duplex is occuring. Auto is already set.

- I am plugging directly into the Rogers-issued router, a Hitron CODA-4582 router-modem with a 1Gbps Ethernet connector

- I have tried using one other cable so far, no change in the test results.

- my testing involves establishing an average speed vis my iPhone 6s plus & speedtest and trying to achieve that same speed or better on the PC.

My ISP here in Canada (Rogers) offers the SpeedBoost compression service, which is likely giving me 150Mbps when it is connected at 100 Mbps.

I'm using speedtest.net with my iPhone 6s plus and this PC to sort out this issue. My iPhone 6s plus has been able to make connections on this same ISP up to 480Mbps. I have copied all details including the test server IP to my PC to help properly solve this issue.

The link between the NIC and the ISP router is reporting 1Gbps, however, the software driver in Device Manager is reporting 100Mbps. I have my Wifi adapter disabled at all times in this testing.

When you say "The link between the NIC and the ISP router is reporting 1Gbps, however, the software driver in Device Manager is reporting 100Mbps", can you take a screenshot of where you're seeing each figure?  I don't understand two different speeds could be reported for the same link.

I suspect I have the incorrect/a generic driver installed when I need a more robust driver..?

Screenshots available in URL www.dropbox.com/.../AAARmikYWL_u5sfKuA39SLZXa

Filenames describe info shown.

I've never heard of this problem occurring using a "generic" driver.  Windows 10 should have had a suitable driver built in that should have worked properly.  You can probably even revert to that in Device Manager by clicking Roll Back Driver enough times to return to the driver built into Windows 10, depending on how many other drivers you've installed since that clean install.

Still would be nice to see those screenshots though.