I think there's really only two more easy things to check. Look for disconnected or deteriorated (cracked or split) vacuum hoses on the throttle bodies going to the MAP sensor on top of the airbox. If those are all good, then it's time to check compression.
If that's all good, the only other suggestion I have is it may be a defective or even just a plugged fuel injector.
If the compression test shows a leaky cylinder, then it could just be a tight valve somewhere. When was your last valve adjust? These bikes are also known to carbon foul compression rings, so a little sea-foam directly in the cylinder might do the trick as well.