if you don't divide the runner from the valve seat to the plenum then you not really getting any advantage , and possibly some disadvantage to this type of intake .
With one round runner, you have two different cylinders with different pulses feeding off one runner. The resonance tuning effect will not work, and in fact become a restriction.
There are true individual runner intakes for the 24 valve, but they are not out to the public yet.
This is part of the R&D process, shops build, and refine over the course of hundreds of manifolds that they build to produce the best possible product. The problems that they find , in every area are ones that most will experience when they try to build their own, and in the long run would have been happier if they had invested in a proven part.
The mounting holes are critical that they do not get in to water, oil of push rods and still hold seal.
The advantages of a runner intake manifold over a plenum manifold are considerable. The advantages are only for the people on the ragged edge and looking for every hp, in a restrictive class. There is still a higher percentage gain to be had with a good plenum intake , over a stock head.
I know that Greg at ZZ has refined his manifolds over the years , and he is doing runner manifolds for some big pulling shops , but these are full on competition manifolds , and the R&D is expensive .
The gains from a good head program, and side draft manifold is huge , I would use it
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