The stock engine is pretty lean...so any improvement in airflow will make it worse.
If you don't want the expense of a jet kit, you can drill the air screw plugs, turn the screws out, shim the needles and consider larger main jets, which is basically a stage one jet kit anyway, but for the amount of work, you might want to consider an Ivan's or Holeshot jet kit.
Check out Ivan's and Holeshot's web site anyway. There are some good tips there. Based on their testing, the stock 2G filter flows better than the K&N replacement.
Synthetic Oil and new plugs are basic maintenance items, but won't increase performance per se. The best bang for the buck is likely a jet kit and low restriction slip on muffler.