Hello! Welcome to the Synergy family and forums!
I have a very good friend running the Pyro 700-52 and the 12T pinion is perfect! My personal ideal head speed is 2050 on 716 mains, I did run 2150 on my Rail 696 mains.
As for acceptable ranges of head speed, that's the beautiful part of the E7, it is such a versatile platform, if can be flown as low as 1000rpm! (See Matt Botos and Rob Cherry's extreme low head speed threads) and as high as you can imagine. I would think 2200 or higher is over kill, but hey that's up to your style.
Good luck on the build, I recommend you look over the Tips and Tricks sections for the E7. A lot of GREAT information! Enjoy the Heli, it's a blast.