They are expensive, but I'm eyeballing the HD Sena 20's. They are the only ones (so I'm told) that will give you true stereo, and the speakers are supposed to be superior to the old tethered style. And yes, you'll get whatever is playing back from your Boom system - GPS audio, music, phone, etc.
If I buy a intercom headset for my ultra limited will I hear music through the headset.
I would say yes, but the quality of the music will depend on what you're planning to buy (wired?, Bluetooth?, H-D brand?, 3rd party?, etc), how you set it up & what you personal expectations are.
If I buy a intercom headset for my ultra limited will I hear music through the headset.
Yes, just remember to route GPS prompts, intercom and media where you prefer to hear them. Any of those can be routed to your external speakers or your headset.