As a current 231 owner and past 20C owner, I have to say, the K maintenance hasn't been bad. Granted, its only been 2 years, but it has been about the same cost as the C. The annuals are easier (One piece belly, fewer inspection panels, much fewer screws in the cowl). Granted, there are 2 more cylinders and a turbo, but really hasn't been a problem. I did have to replace a turbo check valve, which was about $500, but that was a clamp chafing problem, so can't really blame anything 231 specific on that. Other than that, had an alternator problem, which was just a connector, and a dead battery...that's it in 2 years.
As for performance...wow...I don't worry about density altitude anymore. I go in and out of Tahoe and mammoth in the summer with no problems. The 75 gallon tanks are great and a bit of a speed mod in of themselves. The speed is great, the range is great, the systems are great. Much roomier than the C too. The only problem I've had so far is finding places to top off the O2.