I have no problem believing the cast manifold would give you slightly better spool up than the tubular manifolds out there. More volume requires more exhaust energy from higher rpm to increase the pressure in the turbine to spin the turbo. I'd have to say it's give and take though, what you'd gain on the bottom end is probably lost in top end power, at high boost once you have a lot of exhaust energy, I'm sure the cast manifold is a bit of a restriction. At what point is it too much a restriction?
I did a lot of searching to see what people's experience is with this, and I saw only one somewhat definitive post regarding this:
IME, the cast manifold spools 200-300RPM better than the header.
With the same turbo.
Andi
__________________
Andi B.
'98 Quicksilver Supra Turbo, Sold
'04 Millennium Yellow Z06
Boostaholic.com
OneLapSupra.com
That was posted in this thread
IME = in my experience, meaning I believe Andi has tried this, as he was having problems with different manifolds. Hopefully he'll jump in here.
FWIW, relating experience to other turbo vehicles - Turbobuicks have several options of header to go with. For the longest time people would go with Hooker headers, which had a design with a lot more volume, which to the eye, looked like it would flow a lot better. Their stock manifolds are decent, tubular design, but are prone to cracking. It was quickly learned that the Hooker headers were a far inferior design, and introduced a TON of lag. Now, they might have performed better at a higher boost level, or with a more modified car, but for 95% of the cars out there using those headers, they just did NOT work.
If a shop had the psychotic willingness to test all of the combinations out there, I'm betting you'd find out that using the SP budget or cast setup would work well up to 400-500rwhp, then started to become a restriction to top end hp, at which point you'd benefit going to a tubular design (HKS/BL/SP/Greddy/RPS).