Can I improve carb performance without buying a whole jet kit?
You can get about 90% of the performance of installing a jet kit by installing new main jets. Main jets are available from better motorcycle shops. You just bring in your old ones and tell them what size you want, or even tell them you want one size larger (richer) or one size smaller (more lean).
A lot of Japanese bikes come jetted rich on top and will make more power if jetted more lean (the needles will then need to be raised a bit to richen them, as they are usually on the lean side to start with). My race Ninja when I bought it had been jetted with size 98 main jets (stock is 105), and it had lots of power. The reason I suggest buying main jets without the jet kit is that a set of main jets for a Ninja is about $8. A jet kit for a Ninja is about $90.
The nice thing about jet kits is they come with instructions and they are based on a lot of actual dyno research, so they get you in the ball park right away. That's what you're really paying for.
I did some ass-dyno jetting on my Ninja last year. I later duplicated the dyno testing (same jets) on a real dyno. The old ass-dyno picked the same jets the real dyno picked.
One thing to watch out for if you buy jets on your own: DynoJet brand jets have their own sizing system that doesn't correspond to Keihin jet sizes.