I owns 16 ES and I really like the electronic adjustability. I still would like to re-valve and re-spring the shock. That would be ideal. Shock is to undersprung for my tastes.
A re-spring may be possible, but that would result in the shock damping being messed up (too much compression and too little rebound). The one downfall of the ES system is that the fork's compression and rebound damping are not individually adjustable. If I felt a need to, I'd see if there is a way to index the two adjusters somehow to accomplish that.
I'm curious why you think it is under-sprung in the rear, especially living in the northeast, land of a billion potholes.
I would assume that you are looking to quicken the turn-in? Have you tried upping the pre-load to 2-up or 2-up plus luggage to steepen those steering angles?
I find the rear spring is a good compromise for me solo at 225 lbs, and still works quite well (with some added preload) with my ~100 lb wife on the back.