The problem with the sticky weights is the can come off, dramatically changing the CG as you fly. I don't have an Avistar, but most trainers aren't all that sensitive to CG anyway, and nose heavy is always better than tail heavy. (I assume you're testing with an empty fuel tank.) If possible, I'd have an expert fly the plane with the engine mounted as far back as possible, and no added weight. My guess is that with the TT .46 (a very common combination), it will fly fine.