I've tried a LRP Super Sport, a LRP v7.1 and a CS Electronic Rocket Max.
I havent noticed any difference in top end speed - there might be a little, but not much...
However, there's another very important difference: BEC voltage. Sport ESC's usually only deliver 5 volt for your steering servo. The result is a 25% slower steering servo, compared to the 6 volt which is common in most high-end ESC's. A faster steering servo means a much more direct steering. The only exception, is if you're using a servo with a separate power supply wire, which is directly connected to the motor battery.
I also agree with Sosidge - the main difference is in feel. For stock, the 7.1 aint the best - the first part of the throttle move, dont have any effect, since the output is too small. Thus the effective throttle travel is reduced.
What I like about better ESC's, is the adjustability, thus it's easier to adapt to a FEEL of the throttle, that suits YOU and YOUR car. But then again, some of these adjustment, can also be done by the TX. Like initial throttle.
A very nice feature of the Rocket MAX, is that it have a selectable function, where a small amount of power is applied to the motor for a few seconds, after throttle-stick enters neutral. Thus it's easy to take out some of the braking of stock motors, when throttle is in neutral. Also a very nice feature for one-ways
Personally, I find that a good feel of the throttle, makes the car easier to drive. So at my opinion, mid-range to top-end speedos are worth it. But hey, I'm on a budget, so I bought my Rocket Max second hand for less than a brand new LRP Sport