Yes you can upgrade (or downgrade) and tune the HE cars like any other car…
When you do it all the upgrades and tune will be removed but the “boost” will remain… there will be a bodykit upgrade to put it back to the original HE spec.
The upgrade parts that were removed will be available at zero cost in the garage…
Capt I just put 2 different tunes up for the HE Bmw Drift edition feel free to try them out. Had no issue with the tires and you can have alot of fun with it to.
I had a similar issue, but this was on the Ferrari F12tdf. It has sport compound as the stock tyre and can’t be downgraded, however, with a bit of tuning and somewhere in the region of 1,210bhp, it drifts nicely and feels like any other drift car.