Turbos altitude advantage

[Aboutwright]

I'm not quite sure of what you're aiming for in using the Skymaster.
What will be it's mission 80 to 90 percent of the time. Will it be mission critical (delivering human organs!) or personal (you can pick your weather).
I have been a corporate pilot for over 25 years and been blessed to have flown a variety of aircraft including jets. I've flown normally aspirated Skymasters and currently own a P337H. I fly it from northern Ohio to central Florida non-stop in the high teens at 62% to 65% power and it gives me 180kts with plenty of reserve fuel. I can say that the previous comment about the extra power to climb above icing is good, especially if there is high terrain below, but it is only good if your flight is going to take you beyond those conditions or you will just be decending back through it.
Cruising above 10000ft will definitely put you in smoother air above the thermals of a hot summers day and a normally aspirated Skymaster will easily do that, although not quite as quickly as a turbo. Other types of convective activity comes in many shapes and sizes and many cannot be topped by any type of Skymaster!
In respect to the TSIO-360's longevity, I agree that ANY engine will reach TBO more easily in the hands of a smooth pilot and not a throttle jocky, especially the turbo'd versions. Also it is critical for a turbo'd engine to be set-up or tuned correctly to live long and be happy. Items such as unmetered fuel pressure, metered fuel pressure, turbo wastegate settings, and spark advance need to be adjusted correctly in relation to each other, and require a mechanic with the experience and the tools for the job.
But getting to the point. It sounds to me like you are looking to buy your first Skymaster? If that is the case and most of your flights are not long or "mission critical" I think you would be happy with a nicely equipped normally aspirated version. Save a little on maintenance costs now and get to know the breed. Trade up later if you feel the need.