To be quite honest Glyn, most pipe and chip options make very little difference to "performance" apart from added noise. The noise itself can convince you that the bikes making more power and going faster.
I would suggest that most people swap cans just for more noise.

Now fancy cans along with other engine mods such as big bore, big valves, hairy cams etc is a different matter!

Try Remus or Leo Vince cans.
Busters had some in their sale recently.
Typical prices are;
Oval alloy road £199*
Titanium road £250*
Carbon race £250
Alloy race £210
Prices are for LeoVince on the Busters website.Items marked with * are road legal.

[Edited on 23-8-2005 by KeefyB]

Glyn, buy a spare set of standard cans and then get your original modified like mine have been. Only takes five minutes to swap them if you need to.