I'll have to back chris on that one considering I too have the Corsa manifold which I had some input in the final design for the reversed mount.
So far this manifold is coming on sooner (I have a PWv-mount so hard to get exact figures) than my old custom plenum which has a plenum volume which is almost half the size of the Corsa mani. I'm having to wait for the final tune as we ran out of fuel but the car is performing alot stronger and is breathing much better compared to the old setup.
This is made for setups that can push enough air to warrant it. Most people will not out flow a sti manifold. Here in Australia 500 hp atw is the limit of the standard manifold. Unlike in the U.S were u can make that sort of figure on an fpgreen.
There has been a fair amount of R&d which has gone into it. This isn't some cast piece that costs very little money to produce.