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You could end up saving a noticeable amount of weight even if you stick with twin 4-strokes. Mercury has 115 Fourstrokes that weigh 359 lbs in the 20" shaft version. Assuming your current twins are 20" shafts, that would be 40 lbs right there, plus the weight savings from removing the 2-stroke oil tanks and oil (another 30-40 lbs ??) And all of that taken off the transom where it has the most effect.

The Merc 115 ProXS Fourstroke is the top performing version of their 115 Fourstroke family. Supposedly it is generating over 125 HP. The engines are 2.1L, so two of them puts some serious displacement on your transom -- 4.2L! That should get you performance comparable to what you're used to with the Yamahas.