The Twinkie engine conversion thread made me start thinking about what STC-type changes I'd like to see in other airplanes. For starters, I'd like to see what would happen if you retrofit a Seneca II-V with counter-rotating 250-hp Lyc IO-540s.
This would probably be a nifty little performance boost. But I think you'd need about 260-275 hp a side to make a big performance improvement vs. the TSIO-360s. The high power ones I think do 220 hp a side, and the 540s will weigh more.
Or how about a Cessna 340 turboprop?
Here you go:
http://www.onaircraft.com/The-Planes/The-Silver-Eagle-340.aspx
I've seen it, I've touched it, it sure is shiny. They claim 252 gallons total (probably helped in part by the lower weight of the engines).
It seems like a nice package, don't know how it compares to something like a turbine Duke. Someone down the road a ways flew his into a thunderstorm, went from 19,000 ft upside right to 1,500 ft upside down, and decided it was better to replace the TIO-541s with PT-6s. I guess that will help him hit "the goal" next time.
I'd like to put 300 hp TIO-540s on my Aztec, and do a turbo normalizing setup on the 310 (TSIO-520 - 300 hp).
Also would like to see a turbocharged Lancair 360 with a tail that solved the flutter issues. Actually, I've seen that, someone did it in a Legacy, but the Legacy would be better fitted with a TIO-540.
(I think it was 300 pounds - But you could go approximately forever on full fuel, OR take pax and less fuel. Nice to have options.)