unless you're deliberately lowering the bike, you're nuts to use a shorter length shock. Unfortunately the second post makes no mention of what model shock was employed.
You can't source and rebuild/respring a GSXR for much less than a new shock from Sportisi US. So why bother when a proper solution is at hand?
http://sportisimotorsport.us/index.p...ex&cPath=15_17
For the ninja 300, even with my discounts the JRi, Ohlins, or Nitron may be seen as too much and thus I can see the attraction of going with some kind of retrofit.