IO540
I think that you have a point with the prop ice, I once had to do an engine ground test in freezing fog and the size of the ice layer that built very quickly was impressive an would no doubt have had a grave effect on prop performance.
I think that your TB20 has rather a "fat" wing by the standards of the glass aircraft that are now flying, I think that this by be the reason that your aircraft performs reasoably well in ice.
I wish that I had the internet skills to post a photo that I took in the DANYE hold south of Manchester at FL80 there is a lump of ice as big as my fist on the windscreen wiper arm!