Mitch the reasoning is simple.

I had a 94' 28 saber with 500's the reason for 25's and 1.36 drives is because back in 94 merc wasn't building 30,32,34 pitch props for bravos. So you had to drop to 1.36 and spin a smaller prop.

Spinning a smaller prop faster also had to advantages for reliablility. I ran the crap out of my boat and I only lost one drive at 450 hours, the other drive has 600 hours on it now and the guy who bought my boat says it's fine.

Dan