We ran one for a few months back before we had a chip. When you hammered it, it would put out a slight gray haze. Even with a straight pipe it just seemed like something was holding it back. The blue one will turn it into more of a dog. I don't know what affect it had on the MPG. The thing that really made us ditch it for another black one was when winter came around. When it got cold & it was below freezing , it would go into the whole "Hi-Idle & Exhaust back pressure valve thing". But it would stay there for a long time without much affect. Installed the black one & worked like it's supposted to. Retarding you timing in subzero temps on a cold Diesel isn't the best idea.
Still have it for a spare, that's just what I think of them now. I'd only install one in an event that a black one failed.
Kinda wonder what would happen if you had one & put a DP Tuner on Lope Idle...?