I've heard about this, just once though.

I'm sure it's a custom job, but anyone have any idea on how much that'd cost?

Or on the performance pros/cons of it?

Or if that's even possible?

 

cost = heaps

performance, no real point - wouldn't spool up much faster than a properly sized single, and would throw more heat into the engine compartment. You'd get lotsa points at a show though.

Definitely possible, I've seen a pic of a manifold like that.