I think the Y pipe is going to see a bunch of turbulence, and cause more backpressure than a straight back 3in. A straight design will induce laminar flow, and you will benefit from the constant flow velocities.

I've considered the idea of a constriting downpipe to increase velocity, and help spool, but I think the simpler solution is to just spray a shot. Or to have a nicely sized turbo to begin with.

-PHiZ