There are a few things that can be done, all have tradeoffs.
Big bore kit would probably give the most satisfying results. However, durability and mileage will suffer.
You can increase compression, but still requires a piston kit and then you need race fuel.
You've already got the exhaust
kit. Are you absolutely sure the jetting is spot on?
You can go with bigger cams but then you are trading low end grunt for high rpm power.
Increased airflow would be a good thing. Bigger carb, better air filter. But this route could be costly with rejetting/etc.
A dyno tuning session can yield great results, but only if your current setup has room for improvement.
The bottom line is that the modern 4 strokes are in a pretty high state of tune. You might be able to squeeze a few more ponies out of it, but none of the options are cheap.