Slip-On vs. Full exhaust... higher % gain with full system.
Advancing the timing... pro's & con's exist, depends on where you are trying to gain power.
Jet-Kit... fuel is good

One school of thought is "If it runs good... leave it alone." However, I do not typically subscribe to that school of thought.

Assuming you are talking about the '99 CBR 600, my guess is any performance modifications will yeild minimal gains and may introduce more greif than good.

However, if you decide to do some mods...a little power gain somewhere in the RPM band with a slip and a jet-kit sounds reasonable.

If you are doing it for fun, more to tinker than really gain performance, than have-at-it.
Those are fairly simple mods that are kinda fun to do.

Else, my 2 cents says leave well enough alone.
Next thing you know you will have some crazy HP bike that never runs, a thin wallet and a bunch of gray hair.