Anyone have an idea how hot they get?

Stock merc's run about the same as engine temp maybe a little higher it is controled by the size of the bypass holes between the manifold and riser. Ed
the bypass holes are actually in the gasket.

They're Stainless Marine not the stock.

That makes a difference how are they plumbed? Ed

Hey Ed, thanks for the response. Water out of the therm housing to the lower manifold inlet, through manifold, out top of manifold though a hose to the (dry) pipes. The water doesn't go through the manifold to the pipes directly via a bypass like the stock.