I just started racing in August, and spending more on servo's was one of my first lessons.
I picked up an RT5 and a
Power HD DS-090M "High Speed Digital Servo". I mainly race a tighter indoor track and a couple medium outdoor tracks, and on both I could out think the servo. The car felt disconnected from my input, and the wheels would wiggle a little around corners. After long mains, the servo would also be warm.
I then got to lap a fellow racer's RT5 and it was a night and day difference. That night I ordered a
JR9100S, and it it is phenomenal.
That said, I now run the DS-090M in my RB5 SP and it works just fine. Not as much weight to throw around, and smaller inputs lead to more steering angle. Every car is different, and if what you are running now works and your servo is fast enough for you, GREAT!