So far as I recall, Nvidia required that there BIOS files had a valid digital signature which means an overall BIOS modification is out of the question. However has you looked up other 1080Ti variant BIOS's that allow for a higher TDP ? I have in the past grabbed a BIOS files for my cards (assuming they are the same model of course) that have a higher TDP rating allowing more power to be pushed to the card for overclocks. That being said, when you grab a BIOS from another card while it tends not to matter which vendor it has come from (MSI,EVGA etc) be sure that when you find a card with a better BIOS that you ensure that the card and your card have the same PCI-e power connector configuration. I have seen people flash a BIOS made for cards with say 3x 6 Pin connections to a card with only 2x 6 Pin and this obviously will cause issues, so be sure to match up that config.
The other thing to note is that if your card has any other abilities, such as run silent, this can be BIOS specific, as such if you flash another BIOS you can lose the 0 RPM feature for when the card is at idle or low usage. Though if I am honest that is not big deal. The same can be said for any fancy RGB functions, again can be BIOS specific.
Check out Techpowerup for other 1080Ti BIOS files , check here : https://www.techpowerup.com/vgabios/?architecture=NVIDIA&manufacturer=&model=GTX+1080+Ti&interface=&memType=&memSize=&since=
Click on a BIOS that takes your fancy, it will load to another window to show you all the specific on that BIOS, including TDP limits etc.