I have tried a few different motors in my XXX-S and I always end up going back to a Green 3. The P2K2 seemed pretty good for awhile, but I could never get the top end out of it that I could with a Green 3, at least not without losing somewhere.
I wouldn't trust the gearing suggestions in the Losi manual for racing conditions. Those are really just general guidelines; to get it right, you have to gear for your track and your conditions. There is no one gearing that will always work everywhere.