I've thought about this recently as well. Like Applying Jon's voocal ability from the 80's/90's to 2000's-onwards songs. Studio or live performances.

Such a task would probably work better on post-Have a Nice Day era where Jon's vocal decline started to become more difficult to conceal in the studio.

What would be ideal is using the isolated vocals where possible, like maybe the Bounce/This Left Feels Right SACD's or the Have a Nice Day DualDisc?

But of course now, there's lots of really good modern software that isolates vocals almost flawlessly.