I use a 925MG for steering and a Low profile JR 9411 digital Fet for throttle in my Impulse.
Hitec's higher end servos(over $100can) are amazing but i think the 605,645,etc arent that good.
Try to go for a servo with about 0.08 speed(anything faster is uneccesary) and a torq of 100oz or more for steering.For throttle go for a 0.09 or .10 and 100oz or more.