Gloria, I absolutely think tree peonies would do well in MI. Anywhere herbaceous do well trees should, too. I'm in Colorado, zone 4B (according to local nurseries re our local micro-climate) or zone 5 (according to the national zone charts) and tree peonies do well here as do herbaceous and Itohs.
My husband is from MI so we go there often, planning a trip late this spring to the U of M Nichols Arboretum
http://mbgna.umich.edu/peony/. Bet they would have an answer for you from your immediate area!
