I have the same combo, and attempted to balance it with a GP balancer. I encountered the same problem. I had a ton of weight temporarily on the horizontal stab in order to get it to balance. I said to myself: "Self, this just isn't right".

I called a flying buddy who is flying his second one. He told me to put the 5 cell battery in the rear slot of the foam block and fly it.

I followed his advice, and it flys/ lands sweet! This is just another of those RC mysteries. I don't think that I know all the little secrets of the GP balancer.

Good Luck!