Oh yea, you have to use the clutch. If you car was only 500lbs, then you could do with out. But, its still a bike's clutch pack. Beware the RPM's when you down shift! Near redline when approaching a turn means way over redline when you down shift.
Besides, Its a quick shift. Flick of the wrist and a tap of the toe. Practice your heal-toe throttle brake use.
There are better clutch packs and clutch cages out there for our transmissions, that will help get rid of that pesky clutch slippage, but nothing made just for road course use.
Again, its a motor bike transmission. Anything you can do to upgrade within INEX specs, go for it.
See ya on the track!
Pegalis