i have the same setup and i get full boost (21 pounds) before 4000 with a streetport. The boost doesnt just ease in, it hits all at once and hits very hard. I can post a vid also.

 

^Wat Catch-22 said. I'm running the HX40 now with the 19cm^2 hotside. 4" exhaust tapered to 3" at the dp, then 3" single all the way back. Running a tubular racing manifold from Shook Motorsports, Synapse 50mm WG. Stock 9s all the way around, 720/1000cc injectors, Cosmo pump, Microtech Lt8s ecu, small streetport on a S4 motor.

It has a rough tune @9psi running off the WG right now and the boost doesn't lag. Around 3500rpms I get all 9psi and it comes on RIGHT away throwing you in the seat. I'm still waiting on my tuner to get his *** up here to tune it at 15psi. Still thinking of putting in 1600cc secondarys and a bosch fuel pump. Also have an aeromotive FPR I need to install and a Jacobs FC100 ignition box. Aiming for 450rwhp @ 20 psi. Or around 400 @ 15ish. Oh using meth/water injection too.

 

So on stock ports if you are wanting 400-500whp you would want a hx40 with a 19cm^2 hotside or close to that?

 

I'm sure that the HX40 would get you into 400s easy with stock ports if you have the boost at 15psi+