A few days ago, I was trying out a 14S A123 setup on my stock T-rex 600. I made two flat 7S packs to be joined with a "Y" type Powerpole connector I made. It spooled up and was nice and peppy feeling. I gave a small fore/aft, left/right and and quick tail rotor check because I hadn't flown that bird for quite a while. After the check I noticed the swash was tilted forward at an abnormal angle so I hit throttle hold and took it home.
It turns out the elevator servo output wheel would move under load. It probably was a Futaba type and I had it on a Hitec servo. I put one of those beefy red Hitec arms on it and I'm back in business. I'm glad I didn't pick it up to a hover.
I also found that a stock T-rex 700 canopy will fit the dual flat 7S
A123 battery setup with them vertical, so that problem is solved, too.