Is there any reason why we shoudn't set a negative AVX offset in the BIOS? Could that cause any stability or performance issues with MSFS?

Do we know if MSFS uses AVX/AVX512 ?

Yes, I had read somewhere that is uses AVX.

I managed to get my overclock stable without AVX offset, but I remember that once I set an offset the cpu clock was reduced accordingly while running MSFS - so i assume MSFS uses some AVX. However I prefer to have a solid all-core-one-clock setup, as most of the more demanding applications use AVX, like video and photo editing programs, and I see no point in having a theoretically higher CPU clock only to have it reduced when it is needed most. But it depends a bit on your day-to-day usage I guess.