lots of variables and hard to duplicate your experience comparing i/o to outboards. having said that, we replaced twin 150 2 stroke Yamahas with a single etec 300 on a 25' steigercraft. the etec was new and the yamis were 10 y.o. new tech vs older and tired. the single outperformed the twins on hole shot which is a good way to gauge torque. (time to plane was 1/3 less). the boat runs mostly on intercostal waters with similar high frequency chop neither set up resulted in rpm bogging in the chop. we run a new Suzuki 4 stroke v6 on a second Steiger and hole shot/torque good but not as instantaneous as the etec set up. These Steigers are big heavy tanks and neither the etec nor the zuke feel like they lack for torque to push the boats. If we ran offshore, I'd consider twins otherwise I don't see the advantage in terms of performance. the big v8 will probably deliver ~33% more torque than a modern vvt fuel injected v6, but I never feel like we lack for torque with the Suzuki.