Ask this question to ten different people and you will probably get ten different answers.
I have the Volant, and if I was to purchase a new CAI today I would still go with the Volant. It is one of the few systems that is a true CAI; no air is drawn from the hot engine compartment. In dyno tests the Volant has been first in power gain in most of them. If not first then a close second. It is a well made unit that will stand up over years of use.
I have the Volant and the Gibson single cat back system. The two work very well together.
I think the gains in mileage will be very small for any CAI.
Neither the CAI or a cat back system will affect your warranty.