I've always perceived that he was under alot of pressure that he bought on himself to spit as many songs as he could because of the time he lost in jail and the fact that he wanted to out-do his rivals in terms of music.
that was part of it, but a lot of it was because, as people seem to keep forgetting, he was going back to jail. he was out on bail pending appeal at the time of his death, so chances are pretty high he would have gone back to finish his 4-1/2 year sentence. i think this was the main reason he recorded so much while on DR, he wanted to make sure his finances would be secure when he got out. remember, after all of his legal troubles before DR, he was damn near broke.
i do believe, however, that most of the stuff he recorded on DR, the OGs, were how he intended them. not exactly obviously, the tracks hadn't been mixed and they would have still needed some work before release, but they wouldn't be drastically different from what we have now. think of DJ Mathematic's Wu-Tang mixtape and the tracks that were on there from the new Wu album. those were the raw tracks, what we got on the album wasn't much different (some instruments added in, a few touch-ups here and there, etc.). that's the kind of shit that needed to be done with Pac's tracks, it's not as though he just rapped over a random beat, never planning on using that beat for that track.