I have a 725hp NA setup with an IMCO transom assembly and an IMCO SC drive(not the SCX). Everything was new as of last July(had 17.3 hours on it when I put it up for the winter). It also has the stock Mercruiser Drive Oil reservoir. When I would take it above 3.5k it would start to leak outdrive fluid out the fitting where the fluid passes through the transom, below 3k not a drop. I drilled out the hole in the top of the reservoir cap and fluid blew out the top. I put a few more flats turn on the fitting at the back and that helped some but it would still leak. I called IMCO and he said to run the drive oil reservoir much closer to empty then to the fill line to allow for the fluid to expand more as everything will run hotter now and the pressure will go to the path of least resistance. Does this make sense to the everyone ? I have no idea how much outdrive fluid expands. Any thoughts will be greatly appriciated.

I know nothing about those drives, however if the info from them is correct (and it sounds it). I would swap to a larger drive lube monitor container. That way it can contain more fluid, and still allow for 'more expansion' room as well.

Do you have the drive oil plastic fitting through the transom ,,if so these can crack and leak

When the drive lube gets hot, it does expand. On my stock Bravo's mine will move up about 1/2" in the reservoir bottle during normal usage. The reservoir cap on the stock merc reservoir bottle works as a one way valve. I would try what Imco said and run the lube at the "fill" line on the reservoir bottle. If that does not work, I'd look into a larger reservoir bottle.