Slightly off topic here. Unless I'm missing something, it's obvious why the Williams system has a smaller bore than the valver one. I was under the impression the Valver is a higher revving engine, producing power higher up, and the Williams is a low-rev unit which produces far more torque at far lower revs. Smaller bore manilfold helps with more torque. Bigger bore manifold produces more top end.
If you're chasing quarter mile times and outright performance, then a Valver manifold may be great. If you're running a tuned engine, of course the extra bore size will give better results.
However, I can't imagine it makes more power low down as well. And on a STANDARD Williams, I can't imagine it will work as well either. If that is the case, then it will become one of the mysteries of the world as to why Renault decided to design a more restrictive manifold for their flagship homologation hot hatch. Particularly after they'd already made the perfect one already.
Preparing to be shot down in flames :P
P.S. Totally agree with Purplesun. Exhaust design is a black art and dependant on all sorts of engine parameters. And a bigger bore doesn't always mean better.