What changes should I make to a hotlap set to make it a good race set?

The only thing I do so far is put on harder tyres and reduce the camber to make them last longer, anyone got more suggestions?

A couple of things I can think of.
You might want to tweak your suspension settings slightly so take into account the extra weight of fuel you'll be carrying when racing. Probably won't make much of a difference unless you're going for longer races though.
Also, make sure you've got a long enough top gear to allow yourself rev room for drafting. Nothing worse than getting behind someone and realising you're in their slipstream but not going any faster because you're on the rev limiter!

Try lower/higher tire pressures, that usually does the trick for tires. Lowering the pressure normally keeps the temps hot at first, but they will last longer for a race online. Usually hotlap sets are pretty good for racing online, but it depends on the car and what tire type.

What car are you concerned about?

The XFR at Aston Cadet Rev. I've tried increasing the the tyre pressure but they still don't last that long. Lower pressure has more grip but they burn up really quick, thats for hotlaps.

Need to get about 5 more laps out of the R3's.

I always use R2s and really lower pressures in the XFR (although I rarely race it). It's wheelspin that kills the tyres, so just go all out for grip.

Camber is one of the biggest differences, you want reasonably even wear and temps on the tyre for longer races. So long as the set is stable enough you shouldn't need to adjust much else.

It's for the CRC event. Me and Quicksilver have been fighting over the WR and are both racing, he says he has been working on his set for longer runs, could be BS.

I'm just trying to make my set tyres last a little longer cause I know he has a slight speed advantage. At the moment I know that if I push it with the set as is I wont be able to keep up for long enough.