This really looks like a half-assed work when I compare to previous lfstweaks, not only is it more limited but has plenty of bugs that seem random.
So, I set FZ5 ai on the grid, then pick up XRT for tuning, set up values and go race, but I have to hold off all the way because that FZ5 is slow. End race and check FZ5 values, only 290hp 3.6 flat 4. Obviously possible to edit other than demo cars only.
Ok, remove ai from grid and select XRG and set 8L V10, only to find out in game it's only 2 litre V4. This is most likely saloon class restrictment in lfs (max cyls and displacement) and has nothing to do with lfstweak, but as such should be mentioned somewhere.