The standard open induction kits do suck in hot air on a Valver/Williams.
I had a K&N on my 16V and then a Pipercross briefly on my Williams. I was only interested in the noise and I dont think you notice a dramatic loss of performance, except off the mark.
Actually getting a power increase from breathing mods is a very grey area anyway with a normally aspirated car. Having just come from a turbod car where a filter or chip makes a noticeable difference, the odd bhp here and there really doesnt make any difference to the experience with something like a Valver (no matter what people may tell you)!!
I went for a Pipercross on the Williams as I just wanted the noise and I know that the cotton filters can get squashed fairly easily.