y8 is better because it has shorter fatter runners, and a larger plenum. giving better throttle response. it is build exactly like the type-r manifold except on a smaller scale. if we're talking about heads here, thats another story. but stock for stock, the y8 manifold is superior. aka-(type r manifold of the d-series) there is a time when too big becomes overkill, but the y8 is no where near that, and seeing as the z6 HEAD flows better up top, it would benefit more from the y8 manifold, than the y8 head itself.
fyi- i find it ironic the Edelbrock mani assumes the similar design of the stock y8. (larger plenum, shorter fatter runners)