Good news from home, i was able to locate and change the core slowdown value on my FX5900.

I can also change this on the 6800U it seems.
Its just a theory, maybe the card has a operating range of 100° and if it goes out of bounds on either side it will think "WTF??" and slowdown.

I changed the max threshold on this BIOS to 100°, if the theory is right you will able to go down to 0° before card slows.

http://home.graffiti.net/abiosfile/6800UVTHRTEST.ROM

If this all works i can set the slowdown to 0°, meaning the card would only work from -100° to 0° which is truly extreme