Ok... so looking at the cooler's design it doesn't look that bad.

Have you:
1. Replaced the thermal compound with something good such as Arctic Alumina. AS5 or IC Diamond will transfer a tiny bit better but AS5 is harder to clean up and IC Diamond is harder to apply.
--- You will probably want to replace the thermal pads as well unless by some miracle they don't rip when removing the cooler assembly. Even then, new ones would be a good idea.
2. Made sure that the heatsink fins are not clogged.
3. Verified that one of the heatpipes has not gone bad.

I maybe talking nonsense here, but how is the fan wired? Maybe the control pin (if it has one and the laptop isn't complaining when it doesn't read the rpm) can be removed so that it always runs at full speed

Ye I've tried without one wire, but no, it won't go to the max. Fan rpm is controlled via voltage, 2.2v for min spin, and 5v for max. Maybe I'll mod this in the future, but now I'm interested in a software way.

As for cooler design... it's not sufficient for this gpu. If you google for xi1546 overheat it will be all over the internet. A thermal diode is only in CPU so the system doesn't even know that GPU is overheating. but even with the fan on max, it can't deal with this super-hot GPU in some cases. But that will be in my next thread here, about this wonderful laptop, which could be a perfect retro machine if only this heat issue buzzed be.