Yeah, I am currently running a futaba s3305, and it is noticeably slower than the hi-tec 645mg.
46u is right, a good, powerful, fast servo is REALLY the way to go...but it's kinda hard to argue with 'crap!!I'm broke!!'
If you are going to compromise, then look for something that has some more torque than the stock one. If you pick the front end up and swing lock to lock...they go pretty fast. Just setting the wheels on the table will significantly slow them down. A boost in torque is going to give you the most noticeable gain initially. That you can do relatively inexpensively with something like the hi-tec. If you go from slow, sloppy stock, to ultra-powerful, lightning quick response...the difference will be so dramatic, you will crash into everything, and have an awful time trying to control it.