So as it turns out, you can easily change the 90 fps cap in coop, all you need to do is edit 2 values inside WillowEngine.ini

1. Launch the game to make sure the config files have been generated
   
2. Go to Documents\My Games\Borderlands Game of the Year\WillowGame\Config
   
3. Change bSmoothFrameRate to true (from false)
   
4. Change MaxSmoothedFrameRate to what your monitor refresh rate is (in my case 165)

And that's it! The game was working fine for me after that at 165 fps, with some minor physics oddities, but nothing game breaking so far.

I will update this guide in case we notice something gamebreaking.