More than one commment above was directed toward the difference between a road bike and a mtn bike body position, and rightfully so.
"if the rider drops their handlebar height or all of a sudden starts riding in the drops or puts tri extensions on the bars - thereby presenting a different portion of the bum/thigh area to contact with the saddle because the torso is rotated further forwards - I see unsurprising comfort level changes."
I couldn't and wouldn't use my road saddles on my mtn bike. Totally different body positions intentionally.
If you haven't yet I would back up a bit and get both your mtn bike and your road bike proefessionally fit first. Then look at new saddles.
As you drop weight you loose the cushion on your sit bones. So any saddle is going to get less comforatble if you only allow that one issue. Dropping body should allow your body to assume a more effecient body position on the bike/bikes if you take advantage of that by adjusting the bike fit. So you have a lot going on when you start dropping a serious amount of weight.
But seemingly from my own experience the biggest change in how comfortable a saddle really is breaks down ot two things. Does the saddle support your sit bones comfortably. If it does, then how "seasoned" your sit bones are to the physical abuse of a bike saddle is going to make a huge difference IMO.
When my butt is in shape to ride.....which is when i can do a 200 mile day with no real comfort issues I can tell if a saddle fits or doesn't by a few laps around the block.
Every time I take extended time off the bike I can get one any one of my bikes and wonder "what the hell was I thinking" with the saddles I use now. A week's worth of riding, adding time every day and being willing to suffer for the down time puts me back to where I was previous and comfortable in my choices. But then all my bikes fit perfectly because I have made the effort to see that they do. Hope all that helps some.