Yeah, a little bit but not more than 1-2MPG. Throttle response did improve though, and that could have a lot to do with it too. Better throttle response means I don't have my foot in it as far, for as long, to get up to speed. And less throttle is used to maintain speed because of a more efficient combustion process, resulting in more power from less fuel. As far as smoke I can't say I noticed any difference, though it doesn't seem to load up and suddenly "blow" a little occasional soot anymore. Just ditching the cat eliminated most of that though. The biggest reason for getting rid of it is to keep from running that soot back through your engine anyway.
Look at it this way...soot it like really fine dust. And we don't want to suck minute amounts of fine dust in the intake. EGR also has very little oxygen, so it has less ability to burn fuel, making more soot. Fine dust, or soot, reduces engine life, period. Even if I don't gain much or anything in fuel economy, I won't, or shouldn't, wear out my engine nearly as fast, so I shouldn't have to buy a new one or rebuild the one I have as soon.
And that my friend, is money in the bank. I may be able to trade it in a year while it's still worth something. I may also not be economically able to do so and need to run it for a million miles. Either way, if I can make it last longer, gain power, run cooler and have less stuff to worry about going wrong while I have it, I'm ditchin the bitch.
05 2500HD, EC, LB. 4" MBRP turbo back, Bully Dog GT, EGR removed, PCV reroute, self customized intake, AFE MP, shimmed FPRV, lift pump and Cat fuel filter conversion, ice picked, head gaskets and bolts at 130,000 Pronghorn flatbed with LOTS of storage and gooseneck ball, RideRite rear bags, front leveled, Lincoln Ranger welder, torch, Puma air comp.