mouse+pedals are superior. would use it irl no doubt. Is it faster than wheel? Yes, but no. I do use it, and i do get pretty damn competitive on it. It all comes down to muscle memory, grind, and how good you can adjust setups/predict car movement.
I did the work for you. I'm adding two video files(sorry for background music), those are clutch timing comparisions between BC and AC, also AC and AC with BC being used, data speaks for itself. :-) https://www77.zippyshare.com/v/UiI4ovwr/file.html https://www77.zippyshare.com/v/GHpb9L99/file.html
program used: https://www.lfs.net/forum/post/1368765#post1368765
Couple notes:
I got no macro's so BC one varies a lot.
as you can notice, pressing clutch while AC is on, makes your clutch engaged same amount of time or slightly longer, which means, it makes upshifts identical, or slower. And for downshifts, there isn't much difference if there's any, maybe as Isaac said, slightly changes car balance, but seeing how little the difference between clutch timings are, it doesn't seem much.
Adding .raf files for anyone who wants to get them into app to compare themselves.
P.S. For me, this is more for trolling purposes than actually gaining an advantage.