Any overhead coming from objc_msgSend is minuscule compared to the gains from things like better serialization of GPU-bound tasks and not having to synchronize repeatedly with the CPU.

If you're worried about refcounting, use ARC (which you have to with Swift anyway). First, the compiler is very smart about optimizing away retain/release/autorelease calls whenever it can. Second, when those calls do have to be made, they're implemented using vtables, and never hit objc_msgSend() in the first place.