Having my transom replaced in my Baja 36 Outlaw. Coosa was suggested by my builder but I cannot find much on using it in a heavy performance boat. I'm concerned with strength vs plywood. Treated plywood is pretty much rot proof as well. What's everyone doing with the bigger boats?

This about says it all about Coosa. Also, who ever told you that, "Treated plywood is pretty much rot proof", is just wrong. Listen to your builder.......

https://coosacomposites.com/the-coosa-advantage/

I would not use Coosa in the transom. I used it everywhere else. I don't think it will handle the compressive loads near as good as wood.

Glassdave is probably the guy to ask, he has way more experience with the stuff.

Outerlimits has been using it for close to 20 years now. Worlds fastest vee bottom has a composite transom

Curious about what thats costing you if your willing to share

A PVC foam core, such as Divinycell is another option and has been proven in boat building. Pat @ Active Thunder uses this in his boats and the test boats that he builds for Mercury to use at their testing grounds.

How long did the wood transom last the first time?

So, it's like a lighter weight StarBoard? HDPE?

Starboard is nothing like Coosa.

Not long enough!