I wish people would stop saying extended physics is only rain physics. It's like 10 things, if not more (camber changes, tyre heating, cooling, tyre wear, tyre falloff, chassis flex (sort of),....). This are the lut files from a vrc car (picture 1 - 148 in total), and in picture 2 is a normal car from rss with no extended physics (46 lut files in total). So over 100 aditional files just for rain?
Anyway the PM3D cars are around 2 seconds a lap faster. What I did was make a copy of both PM3D cars with car tuner, added all the lut files from VRC (cause I don't know which are for tyres) and replaced the tyres with VRC ones, and added extended-2 to car.ini. I also edited the minimum ride height of the Volvo and the Primera from 0.0045 to 0.0055 (as in VRC cars), and adjusted ride heights accordingly (don't know which is more correct, so I went with VRC). And that is it, all the cars now run within a second of each other.
Meanwhile this is truly BTCC as I remember it from the 90s

. Last lap of a 10 lap race at Knockhill (you have to put the AI at 100% agression. You just have to, no way around it

):