Only some vocal and guitar parts were rerecorded on some of the reissues. The rest is the result of Mustaine's remixing.
I've picked up Countdown and Rust in Piece, and heard some tracks off the rest of the albums.
I like what I'm hearing, it definitely sounds more like what Mustaine intended with his compositions...
But he definitely does not have a great command of reverb. The mixes are generally too dry, and when there is audible reverb, as andy said, it sounds weird.
Overall, good work. More punchy and modern... just too dry.