Ok so my thinking was right.
I can't believe RaceLogic wouldn't allow a diameter difference, it's widely used for various applications, they certainly cannot impose same size everywhere. Another thing I'll try to confirm.
Duh, well after googling around I found a post from 2004 that says RLTC uses tire size setups, tire diameters and car speed to figure out if the wheels spin over your pre-set allowable slip %.
Some people changed tire front and rear for street vs track and it changed the proportional setup without causing any issues. But it is configurable.
Note to self for later % difference calculations: 255/35-18F and 285/30-18R street vs 245/45-17F and 275/40-17R track, no difference in RLTC management.