I would definitely snap up a barrel (preferably shorter 10-11") and ammo if somebody would import the 300 AAC blackout into this country plus ammo (as long as the price is decent, no gouging). The reason I say Blackout over Whisper is because it seems like more manufacturers are coming out with the blackout barrels. I think these would be great sellers for any company that could import them.
Wilson Combat has also started putting out a neat looking 30 Cal round 7.62x40 WT and their own ammo. The only problem is you cannot use your 5.56 magazines, you have to use the Wilson mags.
http://shopwilsoncombat.com/762x40-WT/departments/411/