(In reply to debianlinuxero1 from comment #2)
> Created attachment 123111 [details]
> VBIOS
> 
> The vbios.rom file

Thanks, that's helpful. I'm afraid I also need some info from the official
driver though. In particular a peek from the 0x1002c0, 0x1002c4 and 0x1002e0
registers in the lowest performance level. Could you obtain an mmiotrace of the
official driver going down from the highest to the lowest performance level, xz
it and attach it to this bug please? Alternatively you can nvapeek these
registers in the lowest level (as observed in nvidia-settings).

For reference: I suspect the "high CAS" bit (MR[2], bit 0) in the GDDR3 specs
we used for reference is in fact bogus. Your VBIOS appears to set this bit upon
boot, but apart from the lowest perf lvl, the values written for CAS would
always correspond between the low and high lookup-tables. For the lowest
perflvl, only the low lookup-table contains an entry for the CAS latency. Trace
should help determine whether the official driver unsets the "high CAS" bit or
rather ignores it.