Futaba S9551 and family does the heavy lifting around here as well, and it's a good choice for the price, but if you want something that won't flinch I guess there are some brilliant JR servos with monstruous torque and similar speed. These are heavy full sized servos with lots of substantial metal, but these days the cars are so light they aren't a liability anymore. On the contrary, they help better balance the car.
Team Greasy Weasel
The best upgrade to any car is some driver skill.