Budda, you're absolutely right. And this is a question I struggle with daily. If I were to upgrade down the road, I would be paying about $300-500K more, to go 30-35kts faster. That's a high price for that speed and do I really need it? My maintenance will aslo go up as now I need to deal with gear overhauls and heated windows etc. But what speaks for it is longer range. I would say that's where my main interest in the Jetprops comes from - long range tanks and the ability to go far. Now, there is an old STC to upgrade to drop tanks on the 680V, but it only brings the capacity up to 437gal, which would almost certainly take it very close to MTOW. Range would be in the 1500nm region.
Certainly keeping the 680V, now that everything's been done on it, would be the best option.