Before H-D started installing the Ventilator or Heavy Breather air cleaners on CVO's, they came with the stock plastic airbox and small panel type filter element. In those days the first modification was changing to the round SE air cleaner inside the stock outer cover, and the exhaust system. Now with the Ventilator being standard, you already have the equivalent (or actually better) of what we used to do on the earlier models.
There are a lot of "claims" from people trying to sell you different air cleaners, but little to no facts to back up their claims. If you plan to do more than just Stage I mods and really think you need to address the air cleaner, talk to some of the honest dyno tuners on the site about how much, if any, improvement you might see by changing the air cleaner. You might be surprised. And don't forget the downsides of hanging a bigger filter on the end of the TB; having your leg rub all day on a wider air cleaner can get old really fast.
JMHO - Jerry