its hard to say for me since i love everything from stp, they are my fav band but i would have to say on the N4 record scott and the band sound so live and vocally his range is just incredible atlanta was actually just a demo and they ended up lovin the vocals so much they kept it, but my favorite album is still Tiny music its just so differnt from everything else and a complete masterpiece and underated
I think also what happened was that during N.4 his range got wider so he focused on that rather than the techniques he had during Core and Purple. I think that's where his voice started going down hill by singing all these high ass fuckin chorus on most songs, i mean if you watch Unplugged he doesn't have the widest range but he could use his voice like gold.
I assure you Scott's technique only got better during No. 4. If you keep up with the gravely voice (like he had during core/purple) you're gonna damage your vocal chords. In No 4. he went straight too a classical singing technique (falsetto included), so I doubt going for to much range damaged his voice. Had he not smoked or done drugs his voice now would be more or less just as it was in '99.
If you listen to his voice, you'll notice it's more nasally than it was before. That's far more indicitave of tobacco/cocaine damaging the airways than a damage to his vocal chords. If he had damaged his vocal chords by singing wrong he would have lost his voice and would have to resort to speak singing ala Springsteen.
People go on about how much "worse" Scott's voice is, but it could be a hell of a lot worse. Sure it's a little lower and gravely, but he can still hit a shit load of notes... He's really only lost his falsetto. Imagine him not being able to sing at all.