If the faster people are running lower wind motors (10, 11, 12 turn) it will honestly be tough for you to keep up with them using a 17.
Otherwise, you can always try gearing up a couple of teeth on the pinion, but don't overgear it. Try not to go below an 8.0-8.5 final drive ratio. You can calculate the ratio like this:
(teeth in spur / teeth in pinion) x internal drive ratio
Not sure what the internal ratio is on an HPI Sprint, you may have to check www.hpiracing.com
for that information.
If you need a cheap, lower-wind motor to keep up, you might try one of Trinity's machine-wound Speed Gems motors. They are in the $35-or-so range and should get you a bit closer on top speed.