No it shouldn’t result in any performance difference, it’s only a correction to the way certain part masses/volume & inertia are calculated.
The point of doing this in the first place is to allow you to adjust your game to the change. If it remained optional, it leaves an enormous amount of surface area in the engine for bugs to happen and it creates a lot of extra code in the engine to maintain. It would mean adding the same new feature twice in two pieces of code.
In this case, it would also mean testing new physics content with the feature on/off and painstakingly tuning it until it works for both, and, well, that’s just not feasible. That would then mean that any new features would either not support the old way, or Roblox would need to make the feature change with the setting, and that too is problematic and adds more code and surface area.
I think it’s pretty safe to say that if there were better options Roblox would take them, but, at least keep in mind they’re inputting effort to allow people to adjust first.
I recommend reading this quote from tnavarts: