I know this thread is old, but I'll add my input for someone else's future reference.
I had the Volant on my '06 2.8L since I got it in '06 and took it off recently, as the smog place wouldn't pass me because I'm lacking the CARB sticker.
1. I immediately went from 22-23mpg to 18mpg.
2. The kick-in-the-pants feeling when punching WOT was gone.
3. The engine noise at WOT which, frankly, always embarrassed me when people were with me, was gone. Cab noise is MUCH quieter now.
My pcm is still tuned, I still have the 3.5 TB upgrade, K&N filter, and a Dynamax Super Turbo exhaust. I'll probably change the exhaust, as some baffles must've rusted and come loose inside of it, as it rattles something fierce. I'll probably go with a Magnaflow next. I'll probably also get the pcm re-tuned, as I don't think I'll put the Volant back on. Yes, I will sorely miss the mpg, but my overall requirements for the truck have changed.