When I purchased my truck 95 2500 bolt action tranny all the mods were done for me so I can't comment on the fuel mileage increase but I can tell you from looking at the place where the silencer ring used to reside that I highly doubt it will do anthing for you by removing it because what it exposes goes no where, it's just blocking off a cavity that leads no where meaning it does not allow extra air to the compressor side of the turbo.
Now I did test drive a stock trimmed truck but that was a while before I picked up this one and I don't remember being overly impressed with it's performance - no this one knocks my socks off but she's warmed up a bit in areas other than the just exhaust.
Here is the list of mods on her but I wouldn't recommend them for an autotrans unless your going to beef up the tranny while your at it as I read on many different forums that the auto's don't take kindy to bombing as they call it.
#10 fuel plate (Slide forward some), 3k rpm govnr spring kit, AFC spring kit (star wheel moved forward some), Straight piped except for resonator (no kitty cat or muffler), BHAF (Big Honking Air Filter), Boost control elbow and timing advanced. There are other mods but they don't add to the power level they are for handling the power better and bringing the power to whoa a little easier, along with some creature comforts mods.
But with any diesel (any flavor) letting them breath easier (less restrictive exhaust ie opened up) will add power and mileage. How much mileage depends directly on how you drive but I doubt it will net you the mileage claims that the guy you spoke with says. Getting these things up into the 21+ club is going to hard, I have in my records since owning her (since fall of last year) show one tank where I got 21 and that was just the other week but it fell back down to 19 the following tank so I assume at this point that it's either my ON/OFF right foot programming or I just did not get that last click in there like I normally do when I fill. But overall I'm pulling 14.1 but we have half our mileage at this point with a 10k lbs 31ft TT strapped to her rear and pulling through the mountains of KY, TN and GA. So overall not bad in my book but with load I typically see around 17+ running 70~75 on the highway and laying a little smoke while in town with the occasional tire slippage here and there. The winter fuel this year has really pulled down the mileage - I think the local distributor went a little overboard this year as compared to other years in their winterization of our fuel supplies here. I'm starting to see an increase the mileage but then again this is the time of year for them to start selling "summer" fuel as it's called - meaning not blended.
