I'm sure the Vetus is tight. I know because, before I changed impellers, I tried closing the thru hull, then manually filling the Vetus to the brim. I forgot to re- open the seacock, and when I started the engine the Vetus held tight. The suction made it hard to open.

I'm starting to suspect the SW pump cover and the green residue on the old gasket that indicated a leak near one of the screws. Perhaps with the increased suction caused by the new tranny cooler, it is exposing a small flaw in that area that the new gasket didn't cure. May try the Speedseal, or at least a new cover which I do have a spare aboard.