I do have stock exh. , (no cat. conv. ) but the stock exhaust is pretty beefy. Just the AFE7 air system and volumizer. My pyro probe is pre turbo. When it was stock I could get it up to 1400 up a long steep 2 mile grade in 6th gear doing about 90 mph, empty. It seems about the same with the volumizer. I have noticed better fuel economy, gained about 1/2 mph with the AFE, and about 2 mph with the volumizer box.
The thing I really like about the Volumizer is it is based about 90% on timing and 10% on fuel pressure. Since the 3rd gen. injection system already has alot of fuel pressure, I don't think it is a good idea to boost it much more. I also like the volumizers switching, you can shift from stock mode to +50 HP, to +100 HP on the fly inside the cab.
I played with a powerstroke and a Dodge V10 this morning going up that grade, left them both in the dust, the v10 came in 2nd.