Tongue out of cheek, I wouldn't hesitate to replace ribs, capstrip & sheeting of 3/32" super-dooper-light balsa with the same of 1/16" harder balsa. Ditto for fuselage formers, tail feather ribs, and even fuselage sides if they're meant to be flat. Planking would be hard, because you do a lot of shaping. Clearly, things like big blocks that are to be hollowed and shaped just need to be light balsa, or you need to replace them with molded -- which would require some significant redesign, and would demand 'A' grain, and gets harder as the balsa gets more dense.
(Stepping completely away from balsa for a moment -- has anyone tried shaped foam, with a hard fiberglass or CF shell?)
As balsa gets more dense its strength to weight ratio actually increases -- but there's less surface area for gluing, and less margin for sanding away strength in pursuit of a nice contour.
I weigh every piece of balsa I use, then decide where it's going to go. Ever since I started doing this -- even without consciously thinking about it otherwise -- my planes have gotten lighter. I suppose at some point I'm going to have a huge pile of balsa in the "use only for spars" bin and nothing in the "use for fairings" bin, but at least in the mean time I'm maximizing strength, minimizing weight, and not spending too much extra in the process.