8s a123 = 28.8v fully charged, 26.4v nominal. I can imagine the load he has on it is pulling the voltage down pretty hard... The voltage drop is most likely what he can feel.
I ran a 600xl on 5s lipo. It worked well but when I switched to a 70mm Medusa the improvement in torque and speed was HUGE. It's like Linc says, spin-up is noticeably slow in the Align motor - because it's designed to turn large rotor blades! When I switched again to a 1515 2.5d Neu, the power is about the same as the Medusa but with much lower motor temps.
Save the in-between step and go straight to the Neu!