Mori Man wrote:Titanium , nuff said

You would say that
Seriously though, MM has a good point. The downside to fitting stainless pipes is that you add weight to the bike as stainless is much heavier than the mild steel the OEM are made from. Titanium is lighter, but much more expensive. On a fully faired bike like this, where the pipes are not on show, appearance is less of an issue (the can is another matter

) so I'd prioritise like this -
1. If OEM pipes are not rotten then repaint and reuse
2. If OEM pipes are beyond repair then replace with titanium
3. If you can't afford titanium replace with stainless
Personally I wouldn't replace useable OEM pipes unless I had modded absolutely everything else and I desperately wanted another project. I can think of many other higher priority mods which would give greater benefits. When looking for enhanced performance people often look straight at exhaust / carb / engine work but the ZXR400 is seriously held back by its absolutely toad OEM rear shock. Replacing this for a good, multi-way adjustable aftermarket item is worth probably 10-20bhp for a fraction of the price.