With this bike, you can get where you want to be with very little investment.
If you are slabbing most of the time, might want to consider more street oriented tires, or have another set to switch.
Gearing. Gearing. Gearing. This is going to be your #1 mod. Sprockets are fairly cheap. Search around and see what others are running. I have the 'X' version, which has 17" wheels so my gearing might not be ideal for you, but with the stock 13 tooth front and a 45 tooth rear sprocket, 70 mph is a reasonable cruising speed, topping out in the mid 80s.
Note: this is a lightweight bike with upright seating posture, so you are going to get blown all over on the highway. Just know that in advance...
Performance mods are pretty basic on this bike. Exhaust pipe and airbox mods if you want it louder and slightly more power, fuel tuner if you go that route. Can be had for less than $500. More advanced mods for slightly more low end torque: big bore kit. More advanced with questionable durability: turbo.
Suspension is another area, but can get revalved for not too much money. I still have all stock suspension, but I am 99% street commuter use.
The thing with a 250 is that even if you spend a lot on engine mods (which really only option is big bore kit) you are still riding a 250/290 or whatever you get up to. Not sure what the most HP anyone has gotten, but probably in the 35 to 40 range. With just the pipe/airbox mods, you can be up around 30. Which is a lot of fun on this bike.
The usual farkles like hand guards, different bars, aftermarket turn signals, etc. are typical, but not necessary.