1. Open up the airbox, swiss cheese, custom, or aftermarket. Install K&N filter.
2. Install aftermarket or gutted stock muffler.
3. Install EBR ECM or tune stock ECM.
4. If you want better acceleration at the expense of top end mph install XB9 sprocket and chain for lower gear ratio.
From here is gets expensive....
5. Install big bore kit, head work, and cams
6. Sell bike and get EBR 1190
Exhaust and tuning will yield improvements in all rpms. Remove/disable stock exhaust valve and tune it out. Gutted stock exhaust can is cheapest. Dean Adams is a work of art. Search on here, there are quite a few exhaust makers. I don't think any one is the "best" anymore then one particular auto exhaust manufacturer is the "best". Price, features, sound, performance all factor in.
Search on here for more details of each one.....