Well its hard to say if the belt will slip or not. the bigger cp will make the belt tighter, if it even fits. you might need a slightly larger belt. but if u do just the 3" bp, not the cp, then u might have slip if the belt is already not real tight. ive dealt with belt slip before with the 3.15" pulley lol.
I would say do both the cp and bp at the same time cuz the cp will call for larger belt size, while the bp will call for smaller one, so maybe your current belt will fall in the middle and work fine. you can test it, and if it slips, just get the next size smaller belt thats all. or take off the bp and go back to 3.15". i would at least do the cp doe. more boozt
And did Jose say he wants you to just do aftercooler to see the gains? cuz i think it would make more sense to do everything at once lol. is he the one who wants u to try the 2 different iat locations too??