So far slip ons make more power with less modifications than the full systems. I spoke with Lloyd about the dual Freedom Performance pipes I have and he said that they are seeing some of the biggest power gains with stock exhausts on the Vision. Slip on exhaust has made the biggest power on some of the other bikes.
Dyno numbers of my bike with the true dual pipes back up their statements. Seat of the pants riding it feels more powerful however and head to head against friends it is faster. I am able to pull more distance on acceleration from a stop and roll on acceleration than I did with just slip on exhaust and the bike pulls harder with 2 people.