Nice looking Gee Bee, some suggestions. Since I don't know how you have it set up or fly the plane these are some ideas that I would look into.
Check cg, most of the gee Bees will build tail heavy as a model since the full scale had a lot more iron on the front of the fuseladge.

Do the same diveing loop entry. get your speed up then as you pull the plane to level ease the pull so that you are momentarily straight and level but before you loose too much airspeed. Then pull the nose up gently, once you have the first quarter of the loop started start to ease off the elevator so that you have almost no up elevator and you tend to float across the top. you can't stall a weight less airplane since it doesn't need to generate any lift.
Also in any case are you using a little right rudder as you do your loop? Most guys don't. As the P factor and torque become more pronounced at slower airspeeds and higher angles of attack you will need to carry a little right rudder. About the same amount of rudder as you use for take off.
I think you mentioned lateral balance was good but that is in the static sense and a horizonatl mounted engine can still have some effect in flight.

Its too cold to do any flying now. I'm sure others will pipe in.

Sparky