Keggsey,
Recommend that, whilst you are about it, replace the thrust bearing in the pressure plate - chances are that if the plates are shot then this bearing will be well on its way as well. Check that the clutch push rod is not 'bent' or 'grooved' where it should not be. The push rod sticking is another cause of slip.
Possibly too late now (because you have already removed your old plates), but would have been worth checking whether the 'exposed' slotted portion of the clutch hub as measured from the outer face of the last friction plate is less than the depth of the slotted recess in the pressure plate. This is a good indication of excessive clutch plate friction face wear - when the hub bottoms out in the recess, the load exerted by the springs via the pressure plate reduces and you will get slip.
It is unlikely that the slip is down to a problem with the springs themselves.
I agree with others - reducing the grooving in the basket slots is beneficial - clutch operation will remain fairly smooth if you carefully file them out and the new set of plates will probably last a bit longer than they would if you do not attend to the grooves. There is a limit though and noise will gradually increase. Not tried the ally plates from JHP myself, but will be doing so when the bike is out of warranty and the clutch needs doing again.
As others have said, make sure you get all of the plates in the right order and the right way round (some are dished), etc.
This diagram applies to standard plates for a 996s and a 916sps. The ally plates from JHP will probably be covered by their own instructions/diagrams.
Good luck, Chris.
[Edited on 19-6-2003 by topper]