imo

go as hard as you can on a particular track/setup....on legit road tyres.

Then once you hit a time barrier....chuck on semis and relearn your car. You should maximise your times.

for example...i have hit wakefield 6 times , down to 1.14xx with proper road tyres ( bridgestone RE001's) - i slide a fair abit all over the place but i feel there is a tiny tiny bit more in it. Once i hit this barrier....ill chuck on semis and that should net a touch lower times.