I run a 1/12 onroad. Heats last for 8 mins and I have no problem lasting the full 8 minutes on a 4 cell. You just have to remember, batteries can't be skimped on. Look at the run time #'s, IR #'s (lower is better) and the #'s (ie. 1.18 is the number to shoot for). I use Fusion packs and I last the entire 8 minutes without dumping or going soft. After the race is done, I will start to dump after the 9th minute.
1/12 is a very competative class as well. I run the CRC T-Fource. Smaller, lighter and cheaper than the touring cars. It's the added equipment that will get ya. (Soldering iron, tire truer because the tires NEED to be cut, Charger of competition grade for the batteries you get, equalizer tray/discharger) You're looking at about $600+ in added equipment, depending on brand. But the kit is cheaper and also is pretty much bullet proof. You will break anything if hit hard enough but not as bad as a touring car.
It's a simple design made for racing.