GT4's physics system didn't have the capability of turning off ABS (it affects the physics altogether) and is eight years old compared to what the performance of the physics for GT are now. It's been gone over before in here, and you're right about the tweaking bit. And it's been discussed that it can actually benefit the physics overall if the enhance/tweak the physics to better suit the feel of the physics for motorcycles. IIRC, most people have found Tourist Trophy to be one of the better motorcycle/bike games to date. Obviously, improvements can be made and I made that a point earlier in this post.
The engine certainly isn't perfect in TT. It is quite easy to provoke it into making the bike pull some rather odd shapes in the game and hopefully that can indeed be tweaked out by PD. Seeing that there are only three AI bikes on track with you in racing events, it would seem to be the case that simulating a bike is a bit more demanding. Maybe having a bit more CPU power available in the PS3 will enable a higher level of accuracy as well as more bikes to race against.
Having said that, TT still delivered a very convincing simulation of riding a bike and even if the same old simulation code was reused along with the updated PS3 graphics engine, I think i'd be satisfied. Melbourne House's GP500 is perhaps more accurate, in simulation terms, but I think TT is still the best bike game overall that i've played on any system to date. An updated game from Polyphony is something i'm really looking forward to.
I just wish they'd hurry up with it! We've been waiting long enough.