What is the drawback to the 6S? Is it the charger? One reason I went with it was that I could push fewer amps into it for the same amount of power. With your setup and the motor kv, you would only expect about 8880 RPM unloaded. Now if you are loading it up to 70-80% of that by putting on a prop, it will drop down to 5300 - 7100 RPM, which might be low for whatever prop you want to run. The lower you go, the wider the prop has to be to get the same amount of thrust. My aim was to get the prop up to about 10k when it was fully loaded.
This isn't a subject that is discussed very much on the RCU beginner forum, I have gotten a lot of help over on RCGroups in the Glow to Electric Conversion forum.
Best Regards,
Curtis