Hello mahdlokg and welcome,
I'm not an audiophile (as such - I have a friend who is one and he tells me I'm at about "95% nirvana" and if I spent ten times as much as I do I'd be at 96%) so I may not be the best person to ask but I can relate what I know from reading what others say (some of whom I know and trust, others I know and do not trust and yet others I know nothing about).
From what I can tell, the Japanese manufacturing process and materials (when it comes to CDs) are overall superior to any other country. On average I get the impression that they are (generally) head and shoulders better over any other source (with respect to the hardware and the care in the process). That goes a long way to a "better sounding" (and lasting I guess) CD. There are of course the occasional versions / gold etc that are superior.
One caveat on this - there are one or two of the smaller (obscure) companies that release Japanese minis that have European discs (Arcangelo being the major one and I think Captain Trip?). (There are also the sneaky "official fakes
" as I call them - but I wont go there again).
With respect to (re-)masters and mixes AND technologies such as K2 etc, as far as I can tell there is unfortunately no rule. There is no single manufacturer or even artist that is common across the board (as far as quality of masters... oh okay - a few - Doors and Genesis but then you will get those who argue these are corruptions...)
I had hoped that members of this site would add comments about the audio quality (or source master) of each release but it was always a secondary concern for me and has not happened (much).
Your best bet is to post a question about a specific artist in the yahoo group
. You could also search that group's archive (you need to log into Yahoo to do that).
Hope that helps (somewhat),