I run a 15/45 on my track bike. If you're just riding street, stick with stock gearing. If you want more punch, go +3 on the rear and leave the front stock. But, it will throw your speedo off and you'll need a SpeedoDRD or something similar to correct it. Which is more $$$.
I believe my bike is also 15/45 on my sprockets when I bought the bike used. You will notice the difference in the RPMs when you change your gearing. I bought a speedohealer (easy to install and calibrate with help of youtube and forum) and fixed the issues.