[AlexT]

I'm surprised that the HPDIs are not on your list. Hanging pair on the back is roughly equiv. in weight to four strokes but you'll get more power, fuel economy that is within the range, and relatively proven technology. Doesn't sound like you troll a lot so the quietness of the 4 strokes in that mode is probably not a huge benefit for you.

If weight is an issue I would go with Evinrude's current crop of bigger engines.

With either brand you can go 150 or 175 on the same block so that is no weight penalty for your extra 50 horses. Heck with the HPDIs you could go with twin 200s for the same weight as twin 150s.