Re rigging engine(s)

Let's talk here .. I have 26ft 2001 26 american offshore.. has 98 300 promaxs with sports left/right.. boat runs good 90-95.. iam think going to 250 optimaxs.. for triple the fuel mpg .. I average 2-2.5 on the 300s at 60mph. What could the promaxs drop on sell for? If I goto to 250 would I still be in 90mph range? 300 promax not x was only 275-280 @ prop.. most 250 opti dyno at 275 ish @ prop.. even though about a single 400r losing 150ish hp that route but figured mid 80s still 🤔🤷*♂️.. can I drop optis on my offshore promax mids by changing adapter plates? ..