Looks like Eurogamer was probably correct on these leaked Switch clock speeds they got.
A user at NeoGaf named MDave has a Nvidia Shield console which has the Tegra X1 (Maxwell) GPU that supposed is powering the Switch too.
He ran some tests on the chip, and some interesting things popped up ... most notable the chip will run at a full 1 GHz clock speed .... but only for a couple of minutes (even with a fan). It throttles itself down to 768 MHz after a few minutes of running:
So this is probably the max a Tegra X1 can hit in sustainable performance (without overheating). 768 MHz is the exact clock speed Eurogamer got for the Switch in docked mode.
EDIT: Meant 768 MHz guys, not GHz. Typo.
I believe the same user stated yesterday that the GPU is locked at 641MHz. This is while still supporting various 3rd party games it pushes.