It's hard to say really. Theoretically the emissions crap is bad for air flow. But at low rates of air flow, like cruising down the highway, it doesn't appear to have any effect.
I am a Kenworth salesman and have a few customers that have ran DPF delete kits, I use to drive a 650+ rwhp PowerStroke, I have many friends that have DPF delete kits combined w/ other mods, I have also owned a VW tdi w/ no emissions and a program, and I just bought a Chevy Cruze Diesel for the wife last Tuesday. I have some diesel experience you could say.
My wife's Cruze, that thing gets 50 mpg down the highway at 65mph w/ the AC cranking. That is MUCH more than my no emissions tdi got, about 20% more. And the VW was a much lighter car.
A 7.3/6.0 PowerStrokeDiesel w/ no/ little emissions gets about 13-15mpg. The latest 6.7's from Ford are getting 18mpg quite easily. Once again, I don't quote that one highway trip where the economy was good. I'm actually giving you numbers from one of my customers that runs fleets of these things ran in the field by company drivers, and their fuel cards automatically keep mpg records for them. I'm also combining my personal experiences.
On the wife's new Cruze Diesel, I don't want it to turn in to a hobby like all my past diesels, I don't want it to be loud, I don't want it to smoke at all, and I don't want to give up my 100K warranty. And frankly good DPF delete kits at $1200-2500, on a car that only has fuel expenses of $9125 over the next 100k miles, it would be really hard to ever get the fuel payback. I'd need an honest 22% increase in mpg just to break even over 100K miles (assumes a DPF delete kit, egr delete, programmer, air intake, and gauges for a measly $2K. Unlikely)
Now on my new Canyon w/ the 2.8 Liter diesel engine that I really hope to be able to buy in early 2014, that truck will be a hobby and it will likely get deleted as I care more about absolute performance over mpg pay back. And w/ the truck getting much less mpg than a Cruze-D anyways the payback could potentially come much sooner. But I will be much more sedated this time around (if I will even still be able to obtain a DPF delete kit by then) and will not drive a diesel that black smokes at all nor is loud and clanky.
My wife's Cruze-D btw is freakin' awesome and the exhaust has zero smell whatsoever.
PS. the LBZ Duramaxes are a very special engine. The perfect intersection of the latest technology combined w/ few emissions. The trucks were lightweight and had tiny rims and tires, also a huge factor in mpg. So getting +20mpg is definitely not unheard of in the stockish 2 WD versions while cruising down the highway.