I heard the 6.7 is pretty detuned and capable of much more power than the 5.9. I'm not sure the facts back up losing power because of the emissions. Detuned yes but I don't think there is a flow restriction.
Without the regen running I get close to what my 5.9 got. I think it's important to compare the same driver as we all drive different. When my wife drove my 5.9 she got about 13 to 15 and I would get 18 to 22. Same truck, different drivers. Your friend may be driving like me and you could be driving like my wife.
I have not had any issues with my 6.7 and I just love it. I love the 6 speed auto and the exhaust break.
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2007 2500 4x4 Megacab Laramie 6.7L 6 speed auto, 3.73
2005 2500 4x4 Quadcab SLT 5.9L 4 speed auto 4.10