The 275 is a replacement for the older 350 260hp, I can give a little info on these 357's but I am not sure about the HP differences. Probably just cam and carb. The real scoop on the 357 is the blocks are delivered incomplete as 350's Merc bores them out to 357 this is done to beat CARB rulings on catalytic converters. All completed engines delivered to any marine manufacturer must be equipped with a converter, the risers are rumored to cost over $1,000 each and will have a life span. This will be pushing engine prices to the moon overtime. I got the skinny on this last week when I sold a my first 357, at first I thought they were re-mans why else bore a new block.
Phil
No matter what your beliefs are "GOD BLESS AMERICA"
Fully retired marine tech near 60 years in the biz.