Performance gains begin with air flow. Depending on your budget and local regulations, replace the muffler and air box with better flowing units. Then replace the exhaust pipe with the Hitchcocks 91102 for increased flow. At this point you have exceeded the Keihn ECU's ability to accommodate the additional fuel required. If you are limited to butt-tuning, then the Dobeck Performance Electronic Jet Kit is your cheapest solution. If you don't use a dynamometer, then the DynoJet Power Commander V with Autotune AT-200 is basically your only solution and it's not cheap.
Moving to engine internals, your next step is cams, compression (domed piston) and cylinder head. Guaire can lead you through the ace.cafe offerings.