The differences I saw with the sbd type A exhaust manifold against the standard, is more torque&power from 1800 rpm up to 5.500 rpm. Espesially from 2.200 rpm up to 2.500 rpm, with all gears you saw a huge difference, and with different, better (more metallic) sound.
From 5.500rpm and over, up to the rev limiter I didn't saw a difference at least noticeble to me.
I think if someone have the money is a good buy.
At 2.200 rpm now I have the same power (draw), like at 2.800 rpm with the old vauxhall manifold, and with greater sound!
Also I have a metalic cat from Americat, and a custom exhaust system with 54mm diameter.
All the others are standard if you exclude and a k&n panel filter.