I've seen some H-pipe systems for sale in boating magazines that connect both exhaust manifold exhaust pipe together right before they exit the transom. They are supposed to even out exhaust pules and they claim to add some HP and TQ. Anyone tried these?

High performace cars and race cars have been using H-pipes for years. My '66 Shelby had one. They do increase torque. Don't know how well it works in marine applications though.

We did it a number of years ago, was worth about 100 RPM and is a little quieter.