Running the exhaust separately rather than through the stern drive is a good idea. Stern drives with large amounts of hp get warm, really warm. Yanmars with big diesels have been known to peel the paint
off the upper part. So by routing the exhaust separately that is one heat source less.
The problem with the Mercuriser and volvo
stern dirves is although they have gear oil
in the leg it can't mix so the oil
in the upper part can't reach the cooler oil in the lower part which is immersed in water therefore cool. Result is the upper part of the leg gets warm and if it over heats takes out the bearings.
There is a "shower spray" conversion available which takes a water feed from the front of the drive and sprays it onto the upper leg to try and help cool things down.
I have felt a stern dirve with 200hp through it go from cold to warm in 30 minutes of hard use and that was with a separate exhaust. How much hotter would it be with exhaust gasses going through the leg.
Something to consider perhaps.