Seems the discs in a Z are of good quality, the next best thing is to upgrade the pads & calipers but this can start to cost serious cash. If you go for drilled & grooved discs, be aware you will eat pads quicker than the stock items.
However, you might like to look at this... EBC Red Stuff pads, as (a) they don't have metal in them to leave that horrible brake dust (b) this ceramic compound works from cold as well as standard & bites harder as they warm up
This is where I've found them the cheapest:
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&rd=1&item=180618233789&ssPageName=STRK:MEAFB:IT
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&rd=1&item=170595468241&ssPageName=STRK:MEAFB:IT
and a decent set of rear discs (Mintex) though the guy has just put the price up from £36 to £49...
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&rd=1&item=190537691601&ssPageName=STRK:MEAFB:IT