good question
I dont think the books cover this topic much. As a new Enstrom student, I find the question is relavant to my learning. Your post has inspired me to check and see what Shawn Coyle has to say in his book.....
Just found in Shawns book Cyclic and Collective "load factor plays an important effect on Nr".
Interesting. So I bet the rpm and power change only occurs during the roll into the turn and the roll out. A steady turn would require more power I think.
I have heard that a cessna 150 can only do about 20 circles before running short on power to maintain altitude. Have not tried it.