I've had my E-firestorm for a while now and here are the esc/motor combos I've tried:
1st: Velineon, 7700, 2s lipo - unable to gear down enough - motor got too hot
2nd:Velineon, 8l, 3s lipo - esc would cut out for no reason, eventually one of the motor plugs on the esc broke
3rd: MM, CCBEC, Feigao 8l, 3s lipo - insane geared at 20/50 32 pitch. would wheelie at any speed. couldn't drive it at more than half throttle. it would either wheelie or balloon the tires to nearly twice their size. I did manage to break the diff spider gears with this motor by landing a few jumps on power with the slipper set too tight.
4th: Sidewinder, 5700, 3s lipo - ESC got way too hot. dropped to a 2s but just not fast enough.
5th: MM, 5700, 3s - perfect geared at 17/87 48 pitch. Completely drivable with good throttle control and will easily pull double backflips on jumps.
I pretty much run the 3s when I just want to be stupid. It's way more power than you'll ever need and you have to be good with the trigger to keep the nose on the ground. It really is fast enough on 2s and a lot easier to drive. I'm running 2.2 badlands for more ground clearance although taking turns too fast will tend to roll the truck over.
I also cleaned the grease out of the diff and use 30K diff fluid. Everything else is bone stock. I haven't confirmed this yet, but the Kyosho RB5 spur gears look the same as the HPI spurs and drop down into the 70 tooth range. I'll pick one up and try it out.
Overall it's a great truck that has proved very durable considering that I'm notoriously hard on my trucks. There's a little bit of video of my E at the end on 2s (forgot to charge my 3s) that was ended after the screws pulled out of my MM after way to many bad landings.
http://www.youtube.com/watch?v=aJVkNKYJbck&fmt=18