Mine would do that back when I had the stock exhaust on. Get it checked if you're worried. Those pipes do have a lot of restriction, plus engines tend to get pretty warm when they sit at idle.
Started the R3 when I got home from work cause its sat for the last 10 days, so I thought id start her up and let it warm a little and shoot a little charge in the battery. After 5 mins of idle this is what caught my eye. Should I be concerned?
The Harley fatboy did something similar when new but not all the way down the pipes. After replacing exhaust with Vance and Hines and a good tune it never gets red hot. The Rocket even at idle for awhile have never had the pipes turn red. That is about as lean as it can get.