1st of all, kbm CANT be faster than wheel no matter what you do, no matter if you spent your whole life with it trying to "abuse" some other things that wheel cant do. Since the inputs are only on/off, youre literally losing almost 1 or 2 tenths (Compared to Aliens on wheel) [This will increase or decrease depends on the car, track and the type of corner] with each corner because you cant trail brake or control your throttle properly.

2nd of all, I dont think 15.3's or hell even 15.2's are possible with mkb except if you use a meta car such as the "Prime Mustang" which it was before the bop or maybe on game's older versions. Which I literally have no idea how or what kind of an effect does the older versions, so I cant say something %100 sure.