Tires, tires, tires
What kind of surface do you race on. I'm assuming that the overall tracktion is good if people can run foams. I replied earlier about what tires work good on what surfaces. Obviously there are other choices, but these have worked very well for me.
I can usually get 4-6 weekends of racing on a set of rubber tires with the correct compound and insert. If you are racing these long heats and mains, then go up one step in insert firmness than what you would normally use for a 5 min. race.
This will help your tires last longer.
I like the HPI inserts for just about everything because they have 5 different hardnesses (is hardnesses a real word?) while everyone else has only 2 or 3.
As for your set-up, I would run (0) zero toe in the front on a HPI car, with -0.5 to -1 degree negative camber in the back. Put your camber link in the middle hole on the rear bearing carrier.
Adust for 0.0 to -0.5 degree camber in the front and use the stock camber link location in the front. Use any more camber in the front or back and you will lose grip at that end of the car. HPI set-up is a little different than other cars. I have raced HPI nitro cars for over 3 years. Try it and you will see what I mean.