I've never heard the Piece Of Time EP, but since I'm not a big EP fan, and I already have all those songs on No Limits anyway, I never really bothered looking for it.
As for Sons Of Thunder, I didn't have a major problem with the final mix. It wasn't a fantastic mix by any means, but it didn't hamper my enjoyment of the songs, which are very good IMO.
That said, the Neil Kernon mix is definitely sharper and more precise, in the sense that the clearer separation between instruments makes for a less muddy, harder-hitting overall sound. Also the keyboards are less up-front than they are in the final mix. So it does sound different. But I also don't want to give the impression that there is a *huge* difference.
In some parts, there's actually not that much of a difference between the 2 mixes, while in other places, like the intro of the title track, it is mixed rather differently. While I'd rate Kernon's mix better overall (mostly because it's simply sharper), I can actually see some places where Olaf or the band might have had a problem with it. There are indeed some sections where the final mix comes off as more "melodic".