Well you need to remember that if you replace 2 medium strong and medium fast servos with a really strong and slow servo.......your steering response will suffer. If you buy a GOOD single servo....you WILL find an upgrade in response with a single steering servo.
Running 2 at once isn't a bad thing, unless they get out of whack from each other. They both tug on each other every time you turn the wheel, which tends to cause them both to work harder than they need to at times. Unless of course you get them both PERFECTLY setup and balanced with each other....which will require adjusting over time anyway to keep them that way (slop happens).
So if given the choice of spending say $40 each on 2 servos....or $90 on one really good one....I'd go with the single especially for the Revo. I run a cheapy Tower Pro, but it's plenty strong...just a bit on the slow side. If I were racing it...I would want faster steering response...but the Revo turns like a dump truck anyway....so it's not a HUGE difference.