Interested in any thoughts here too. I do the gym ME for ski mountaineering including heavy packs and approaches. For sure it helps, not just on the up, but also for leg burning descents. My approach so far has been to do a long gentle gym ME progression and add in a small handful of harder water carries towards the end of the cycle before tapering out in advance of goal trips. My go to vertical ski training run is not quite steep enough, I am thinking of changing to adding a few bootpack based steep water carries later in the winter this yr, and just keep with the gym progression as the bread and butter.
My sense in optimization is that the workout that gets done is the better one. Time efficiency using gym makes sense.