You sound like a man speaking from experience. For what it's worth, I won't be divesting myself of any of the original componentry. If one of my "improvements" turns our not to be, I'll just take my time polishing up the original kit before I put it back on.

The shifting thing is interesting to me. I went to Park's website to research how to tell if an RD is worn. I was expecting it to be mostly about front to back spring stiffness. Instead, they mostly focused on lateral play. And a lot of authors seem to claim that short cage RD's shift more crisply which sounds related (longer cage = less lateral stiffness all other things equal). That said, I would have thought that quality shifting would be mostly about the upper pulley being positioned well and the lower pulley would mostly just be along for the ride. If the lower pully has some play in it, so much the better. Viewed from the top, most gears are cross chaining to some degree which means that there's always going to be some torque on the pulley cage, short, long, new, or worn..