I have heard that ppl who have changed from the pipercross to BMC CDA have felt a difference, to be honest I dont rate the pipercross viper at all, sorry. So in my opinion as long as you get rid of your pipercross your doing the right thing
You do have other choices like the Green's RamAir, or the Procomp kits though the Procomp kits are alot more pricey though do show the best gains! just depends if you want to spend £180 or £300. At the end of the day these are all very closely matched so doesnt make any real difference which road you choise as it comes down to personal preference in the end.