Paul asked me to chime in on this so here goes.
I had a 100mph offshore boat and now have a triple engine CC so my experience is based on what I went thru.
First you state efficient, my CC get 1.5mpg at 45 mph which is way better than my Outerlimits got. I dont have perfect data but I had both in the Miami to Key West poker run(different years of course), the Outerlimits could just make it from Miami to Marathon about 240 gallons the CC went from Miami to Key West and back to Miami on 320 gallons. The salt water benefits of the outboards is huge compared to the I/O's, not to mention clean up on each boat, with a CC just hose everything down no worries.
Before I bought my supercharged outboards I checked with our local marina in Florida. He was working on a coast guard boat with triple 300 Verados and I asked if he did all the work on them and he did. I asked if they had issues he said none, this one was getting some plastic piece replaced by the intakes and that was the only work he had done to them and they had 1300 hrs on them. Also you mention high octane that was a concern for me also cause I run the Bahamas and fuel isnt so easy to get. The 300 verados do require 91 octane but have a knock sensor so you can run 87 or lower thru them. Very little to no warranty? I have a 5 year warranty on mine. You also mention pushing speeds not for its original application, maybe true for the older boats but anything designed in the last 5 years the fisherman need to be the first to their spots when in tournaments so they are designed to run fast and efficent.
Bottom line I enjoy boating again no headaches from breakdowns or the noise police, I can now take out 6-8-10-12 friends on my boat and have fun, we will never be the first to a card stop( Boyne Thunder doesnt count Paul) but I always finish the run. And just for the record my CC probably spends 75% its life fishing.
