Most LiFe batteries I have tried in the past have discharged poorly. Even those rated at 20c I tried didn't actually give that they averaged 12 to 16c. Where most lipos rated at 20c actually run slightly higher. C rating is just as important as voltage look at the difference between a 9.6v nimh and a 7.4v lipo.
To be fair I haven't tried LiFe for a couple of years so they might have improved.
If any of my guns required more than 7.4v for instant trigger response and at least low 20s rps I would class it as broken and fix it, so higher volt batteries are not really my thing.