finally took the stunt trainer out today.

Roger, you can hear the engine working overtime as you climb. Do you hear this from the center of the circle? This is certainly telling a story. The engine is being overloaded. This is not a good sign for maneuvering. As certain flying styles progress, engine condition and reliability is ever more important. Slab wing planes are going to always suffer and the engine is doing it's best just to keep the plane flying let alone stunting. What nitro are you using? That can assist a bit if you up it a little. What prop is currently on that plane? If it's a 6" prop, your going to experience exactly what's happening. Reducing your prop size allows the engine to run it it's higher rpm's which benefits it's stunting ability. Rather than drive in 4th gear, your going uphill in 2nd. Slab wing planes need to be greased into maneuvers. This means a lot of arm movement and little wrist movement. This keeps the speed consistent. If you pull full up elevator into a wingover or a loop without arm movement done properly, the plane is going to be strangled. I would recommend a 5x3 prop like a Cox black. If that's what is currently on the plane, I would try a APC 5x3. These however are expensive and will break easily. The APC 5x3 is around $4. I have to ask what line length are you flying as well. I haven't paid much attention to these posts so I'm not certain if you mentioned it prior.

I'm not a fan of rudder offset or engine offset. However others choose to fly is entirely up to them but I will say that both of these compound the problem of speed falling off. Yes, they work for what your trying to do, but it's speed that the plane needs to fly correctly through the maneuver. Down low, you have line tension, you get up top and the speed falls on it's face.

You must first start with assurance that the plane is flying level correctly. If the wing is flying tip high upright, it's going to come in as you climb.  Next you must not have too little or too much tip weight.  If your confident it's good, moving the leadouts back will cause the plane to fall out of the sky even more. It sounds counterproductive to move them forward, but it is the proper way to gain line tension overhead as your not stalling the plane overhead when the rear end is yawed outboard. The plane should execute the same line tension down low as it does overhead. Again, when you strangle the plane, speed falls off, line tension falls off.