To be safe I usually set my cg at the front of the cg range for my maiden. Normally a little nose heavy makes for a more stable predictable plane. Not so in this case, at least in my experience. The plane was squirly with the cg forward. It snapped out of loops, and snap rolls even at high speed were more like tumbles. It was really wierd, it acted like the cg was really really far back in the plane. I landed it and moved the cg back to near the back of the cg range, about 60mm and all the bad habits went away. It tracks like its on rails, loops are straight with no correction needed. Snap rolls were now fast, even violent but predictable and controllable. Knife edge had almost no coupling just a touch of aileron to keep it straight, just a touch. It's become my favorite plane!
As far as Cobri gear boxes you can find them here.
http://www.e-flightline.com/
I am lucky to have a dealer locally. Just throw out the E-Flight gear box, it's not worth the headaches. The Cobri fits perfectly with no modifications, has a ton of gearing choices, is nearly bullit-proof, and is super quiet. I won't use anything else. I've put 300 watts though mine without any problems.
Other thoughts. On the pull pull setup, the white double sided control horn that goes in the rudder had some little ridges inside the holes drilled in it. They made the rudder click when moving it back and forth. I took a real small round file and gave it 1 or 2 real light strokes. This smoothed out the holes and the rudder is like butter now.
Also be careful on the amount of elevator throw you have on your maiden. This plane has a ton of control surface throw, and you can over due the elevator if your not careful. Go easy on the elevator until you get a feel for it. Mine is cranked up to nearly 45 degrees up and down, great for hovering but really aggressive for normal flying. You can't have enough rudder or ailerons in my opinion. Both are very predictable even at extreme throws.
Good luck on the maiden!