I think the Cosworth bonnet louvers are a bad design anyway as the nearside one is right above the coil and the offside one above the cone filter (Admittedly they had airboxes as standard)

Back to the question I don't think it's worth it imo, it may aid induction, but temperature is irrelevant as the air it will be heated up anyway (Hence why you have a decent intercooler)

Also, as you say, if you use the car in rain it will get covered in spray, in Ronnie's situation it's probably isn't an issue as it's used for track etc so far as I can make out?

Martin