No you're right mate. Once you complete all races in classes c - a (i.e. get a 3rd or better) you unlock the S class. That opens up the performance points system and quick tune option where you can alter bhp, weight, gear ratios and things like damper / spring rate etc too.
Not quite the full on tuning of norm in GT but a fair amount for a preview GT game. It makes the online races where you have points restrictions a bit more interesting. It certainly makes a difference as to how well you do in races if you have the car set up for cornering over straight line speed etc / acceleration over top end and so on dependant on the track in question.
Oh, and of course the penalty levels come into play too, soon stops you cutting corners or ramming stuff even if by accident