I was looking into taking on a Formula 233 project. It has the 280 TRS package.

I was thinking that there may be some merit to filling the lower unit exhaust cavity with a strengthening engine block type filler. Using thru-hull exhaust, of course.

Would a good adhesive based block filler give more strength and cause less distortion of the lower unit bearing support sytem?

The extra weight would be negligible, for the potential benefit.

This could be for any prop shaft exhaust outlet style lower unit, that has gone to thru-hull.

Thoughts?

When the lower case breaks it is usually caused by a tooth breaking off and going between the gears driving the case apart. I don't see filling the case helping that.
filling an engine block gives support to the cylinders.