I,m using the 550e on a 16 OZ plane(flying weight) using two 830 3 cell Li-poly (thunderpower)& 11-4.7 slow flight prop. this combo will accelerate out of hover on the up line easily and with great endurance (20 min typ.) or until I crash (LOL)
I found the 11 4.7 slow flight prop best of the props I had for thrust. better than thin electric
11-7 sounds too fat I bet your lugging it too much
also my castle 25 is set up for high pole count and high resistance
I find the 550e a great gearless powerplant, durable and efficient