My experience with sipura's has been that you can get much higher quality audio than normal POTS with certain codecs. I have never used Skype, but I can tell you that it all depends on your bandwidth and codec being used. The only reasons that come to mind for quality loss when having more units, is that either your pipe can't handle the amount of connections, or your QoS is misconfigured.
Edit: Sorry for bumping an old thread, but I saw like 4 guests viewing it.