You wont gain that much if your using a stock spring. Good ball park figure would be bout 5-15fps and perhaps 15ft more range with a bit more accuracy.
I'd highly suggest the Prometheus 6.03mm tightbores. Also make sure your barrel doesnt wobble inside the metal outer barrel, that bit of free play is what causes your shot inconsitency.
Tho, IIRC, Kokanee on this forum deals or did deal with 6.01mm tightbores.
You can find all informantion regarding the 6.01mm tightbores right here.
http://www.deescustoms.com/prod_custom-barrels.html
Just be sure to use high quality bb's with it. I as well as MANY others will recommend Airsoft Elite brand bb's.
Also, be sure to pair your tightbore with the appropiate cylinder so the volume of air being sent through your barrel matches the barrel length. Tho I've found, I get optimal performance, range and accuracy with AK47 length tightbore's, when I pair it with a Type-0 cylinder (M16/Aug - 500-509mm barrels.)
If you have any further questions feel free to send me a private message and I can guide you through the entire barrel upgrading procces.
I hope my information is of use to you.