I've not tried the proper Cardas jobs, but I did try using kitchen foil to try and form a similar screen. On my old Pioneer A400, if the input selector is on an usused input, and you whack the volume up, you get very noticeable hiss from the speakers. Sticking a pair of shorting plugs on the same inputs dramatically reduces this. Using kitchen foil, and manually holding it in place to try and ensure a good contact, made no descernable difference. Naively I'd have assumed that the foil would have performed similarly, although not as well, as the Cardas caps, but maybe I was being optimistic.
The shorts and the caps do perform slightly different roles. The shorts fix the signal potential to 0V, whilst the caps leave the input floating whilst providing a better screen from RFI.
Of course what really matters is how the noise from an unused input affects the signal from the ones you're trying to listen to, but I need to do a bit more listening before I comment on that.