I don't see a smoking gun. You say you can still reach cruise speed (or is it WOT speed?) with no unusual rpm changes. The forward psi, hot @ 150 is plenty to squish those forward clutch plates together. Nothing here suggest low pressure is the issue, at least to me. There is not a total, positive seal at the forward clutch circuit, since there are small pressure relief bleed holes and the fluid path involves the shaft with its own metal rotary seals. And, being hot, the oil is thinning. I think the shop will tell you this is a reasonable pressure at cruise. The next possible areas are that the reverse system is dragging (in forward) or the external oil path is somehow limiting the flow.
The prop is stationary in neutral, correct?
It might be possible that the reverse oil feed is not adequately shut off by the control valve when in FWD position, but I'm totally on a reach here. I tend to think that the external cooling is NOT the issue, since you can get overheat conditions in a just mile of travel (3 minutes?).
Get them to loan you a different transmission??
I'd call Transmission Marine in FT. Lauderdale and/or Hale Marine in the NW. They are supremely knowledgable on these.