Mercury actually had those at SEMA in 2015. Seems like it has taken a long time to get to market - and it is not clear what the availability actually is. Valves on the 2-valve LS7 are huge, so even if these don't breath any better, the lighter valves should open up the reliable RPM range.
But that engine is pretty wide across the valve covers. May have some fitment issues in some cars.