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I like the OHV designs because you don't have some pesky pushrods where I'd like to see the intake and exhaust ports. I also like the 4v heads because they make a nice efficient penta-roof combustion chamber. The peak power output of an AC engine is probably more strongly related to the rod/stroke ratio they would have to use. They aren't terribly high compression for running 100RON fuel but I suspect they have a good amount of dwell since piston speeds aren't going to be an issue.
I'm not a pilot but I do spent a lot of hours in anything from a 182 up to a twin otter. The piston AC do run disgustingly rich. Can't comment on the turbines too much but I do know they sure drink the fuel!
On the diesel conversions I know the C-182 skylane conversion has an STC. If I remember correctly the TBO is about 3000h! Although they produce less power and are slightly heavier they do consume less fuel and the 230hp or so that they make surpasses even the 260hp lycoming above 6k. It's been a while since I looked at the specs but I seem to recall that they used a turbo to keep the power up. Ultimately the climb with max weight was faster than the 260hp to 10,500'. At which point we (the payload promptly open the door and jump out to lighten the aircraft so it can descend again and repeat the cycle all over :P
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