Without fixing the tyre wear, any changes to BOP is just like patching a sinking ship. It's hard enough to equalise cars across all tracks and driver skill levels, but when you add unrealistic tyre wear and the advantage some cars has it's impossible to balance without making the performance extremely lopsided in non tyre wear races.
Gr.4 cars are the most difficult to balance because of the variety of drivetrains (FF, 4WD, FR, MR). Honestly I think if you take out Alfa/GT86/Megane Trophy's OP tyre life PD already did a reasonably good job. The only car that sticks out in need of buffing is the 650S. Veyron is an interesting car, but often forgotten because it has no Gr.3 counterpart so I understand if PD didn't invest too much time on it.
Gr.3 cars without tyre wear are actually pretty close already on most tracks, given a good driver that is able to adapt to each car's handling characteristics. Of course, some tracks will naturally suit more cars than others just like in real life, and I think this advantage should remain. As long as there are a decent spread of tracks in the FIA Championships the results should balance out in the end. What I'd like to change are suspension and LSD settings on some cars to make them more driveable and accessible to non aliens (Ferrari, Alfa, Huracan, R8, Pug VGT). Top speed wise, I agree the McLaren F1 is too OP. If they give it more downforce that will improve the handling while lowering the top speed more inline with other cars. Subaru/Mitsu/Mazda/RCZ/RS01 are slow on straights but they have strengths in other areas so I think they're ok.
Gr.2 they either need to split into new/old, or the old cars need a massive bump in handling. Given the generation gap there's only so much suspension, LSD and downforce tweaks can do though. I'm more in favour of splitting the class because taken separately these cars are among the most well balanced in the whole game. Super GT's governing body really did their job properly, unlike PD.
Gr.1 again needs to be separated to 3 classes: Group C, VGT and LMP. As it stands, Group C cars have overwhelming speed (R92CP mostly) but poor handling. LMP-H dominates everywhere else with only 2 VGTs that can remain somewhat competitive (LM55 and Alpine). It's the most lopsided Gr class in the game and I don't see them equalising cars spanning several decades with BOP alone.
Gr.B is often forgotten but oddly enough they have good balance here. The only one that sticks out is the old Quattro having worse handling than the rest, which can't be helped really. Should've made a fictional Gr.B Audi TT instead.
N class, whew, I don't know where to start. It's basically like Gr.4 but even worse due to all the variety in drivetrains, body styles and years. Unfortunately I think it's difficult to achieve parity and there will always be metas in each class. I think PD needs to lose the N/Gr.X category completely and just treat them as individual cars like in previous games. If they want to use it in a Championship race, it should be one make OR they need to do a BOP specifically for that track and category on a race by race basis. Limiting races to similar car body styles like in GT League could work too (e.g. only hot hatches, K cars, muscle, vintage, sport sedans, modern supercars, etc).