What prop are you running. That engine should have plenty to pull out of hover. Too much prop will kill the performance on a piped engine. No more than an 11-4. I have the same setup for my MTK but haven't got it built yet.
If all your incidences are right and there isn't any noticeable down trim in the elevator I have no idea why it would do that other than CG. On second thought, if the ailerons are offset from neutral they could be creating asymetrical lift while the elevator is trimmed to neutral. You could experiment by raising them both a degree or so and retrim for level flight and then check the effect inverted.
There are a lot of possibilites but I'd still bet on CG. Does the elevator get touchy on landing? If so that is another indicator of being tailheavy.
Good Luck,,,basmntdweller

My MTK is third in line on my building bench.