The thing is that in many cases those are basically swapping a crossfire top onto a manifold that has MUCH larger runners... unless you change everything at once you're going to end up with a mismatched combination that won't run very well.
Secondly, that link does a lot of things that either don't help at all or hurt, like there is no point in opening up the port openings like that, it's kind of hard to see in the pics, but the intake runners run flat in the manifold like the head ports, and the big flat area where it bolts to the head is there just to seal the ports. If you look at that last pic posted, there isn't much metal behind it, and if you open it up to a typical head gasket opening you can only do it at a the flange, the rest of the port stays that small, and even if that wasn't the case, the air flow would have to take a hard, downward turn to use the extra cross section. In the end you're just ruining something that was working and in the process getting something that doesn't work in any way.
He also cut back 4 of the 8 runners... shorter runners might help up top if you increased cross section significantly, but they didn't, and left a square edge that will hurt airflow and fuel distribution, while leaving the outer runners pretty much stock also, further unbalancing things.
There just isn't room in the stock manifold to make it a high rpm breather, cutting out the EGR works, radiusing the port entries works, don't touch the exits (at least don't change their shape), clean off all the casting flash, cutting out the sides of the swirl plates and plenum spacers work also (gives more room for the air to change directions).
I can post some pics later if I can find them/have time.