I have used the hobbywing justock 45a esc (apparently really good esc for blinky stock) and a reventon pro 120a esc and noticed no difference.
The amp rating on ESC's tell you how much current it is happy with passing through before things overheat and fail. The WATT rating on you motor determines how much current it will draw.
Lets say your 17.5 motor is rated at 250 watts. It will pull almost 30a at full charge and 39 when the battery is almost dead. So the justock 45a and a 5000mah 10c battery would cover that with a little room to spare for efficiency losses.
C rating isn't really as important as how long your battery will go before voltage drops off. Less decline in voltage will make you go faster toward the end of a race.