Has is just suddenly started doing this or has it always been at 1500? If it's recent then you might have an air leak around the intakes somewhere (try spraying WD40 on one connection at a time to see if rpm increases) and worth checking the fast idle cable in case it might be sticking and holding the rpm higher if it's not releasing properly?
My GSA idles at 1150 on the GS911 but looks more like 1350 on the clocks so don't always go by your rev counter.
Either get hold of Twinmax gauges or make up a simple manometer (plenty of advice on ADV Rider), you can carefully adjust the BBS without but everything works better if it's balanced properly
If there are no air intake leaks and the fast idle cable and throttle cables are not sticking, then the idle can be reduced by screwing in both idle by-pass screws by the same amount.