Yes, I would try to pressure test it before disassembly, even before pulling the water pump off while the gearcase is off the motor. While it is true that all leaks don’t necessarily show up on the pressure test, most will. Be sure to turn the shafts, and move the shift shaft up and down while testing… It is always better to pinpoint the actual leak before ripping the gearcase apart. Immersing the gearcase into a bucket of fresh water while pressure testing will help pinpoint problem areas also.
Once it is apart, have a close look at the prop shaft and driveshaft to ensure they are not "grooved" where the seals ride. Have a close look at the upper bush/seal housing as well to make sure the driveshaft shock has not expanded, the housing will usually show galling where the top of the driveshaft has been rubbing against it if there is a problem…