is it a 200 or 180 horse arrow? i think its easiest when performing maneuvers to just leave you're rpm's at 25.. especially in ur CFI ride, you can explain how keeping your rpm's at 25 will reduce the workload for a student new to the airplane... i always run 18"/25rpm in my arrow (180 horse) and it puts me right at 120mph... when starting your bank for your steep turn i would put your mani up to 20" and that will keep you at 120 around the whole turn...
like other guys were saying, it seems like you're adding pitch in soon. try not adding any pitch at all until your established in your 50 degree bank, just roll in a little quicker than normal and see how that feels.