Well, generally, you can look at these numbers to get what you want performance wise:

50 watts per lb. = low powered, slow scale flight
100 watt per lb. = spirited sporty performance
150+ watts per lb. = strong vertical performance, with hovering capability

Your original estimate of 300 watts would only fly a 3lb. plane max. I'm running nearly 500 watts in a plane that weighs ~33oz. with 2100 mah lipo, and it is a screamer vertical wise. 1/2 throttle hovers. 3/8 throttle cruising.

You are going to need around 700 or so watts to get 3D performance with that plane.