You didn't say if you were using a separate BEC. If not the money spent will be well worth it. A CC Phoenix 80 with a 6S pack is at its limit as far as the BEC is concerned. I always use a separate BEC when using even a 5S pack. It does sound as though the motor is being pushed too hard. I would check out some of the Scorpions or Hyperions for more power and efficiency. with an 8 Lb.+ plane the power output looks a bit weak taking into consideration the motor efficiency which is not all that great in the E-Flight line.
I just ran your plane through my Ecalc with a Scorpion S-4020-14, 6S, and 14X7 APCE prop: This motor is rated to 80A/1500W.
50A
1131 watts created
886 prop watts
110 watts/Lb.
144 oz. thrust
At this current draw, the motor is not even breaking a sweat and still provides plenty of power at very good efficiency.
You can tell I'm somewhat biased towards these motors but I have been very impressed by the quality of workmanship and performance at the pricepoint htye are sold at.http://www.innov8tivedesigns.com/
Hmmmm, I may just have to build one of these myself.
cheers