Either of the carbon airboxes will give similar results mate, but "bodging" one to fit aint the best idea, so either a Viper of CDA is the one to go for...
I have a Viper on mine and there is a definate performance increase with it fitted, and theres definatley more power & torque in the mida range, as we proved on the rollers last year.
However, the CDA is now a lot cheaper (£160 I believe) and just as good (some say better), so if I were you Id get a BMC CDA!