Do the tests, but I don't think that's unusual. I have virtually the same air horn (Stebel Nautilus) on my Miata and it dims the lights noticeably sitting in the garage with it idling. Anything that draws that much current on a small capacity charging system is going to cause a voltage drop, no matter what.
As MRkeys said, it's possible the unit is drawing more current than it should (ie, if the motor is binding or dragging) so you may need to do a current test on it. Will likely take special equipment since most meters only go to 10 amps.
2006 Vulcan 500 LTD
2013 Suzuki V-Strom 650 ABS Adventure
Last edited by Gary W.; 08-11-2013 at 01:03 PM.