The reasoning is that a less restricted exhaust will provide more power and better economy. I have not heard a lot of good comparisons of mileage before and after of people changing over to a straight pipe. It is obvious that using the federal exhaust manifold and changing it basically over to a 84 300D would increase mileage. The 85's have a different rear end ratio too that helps keep the rpm's down on the highway. The straight pipe doesn't have the filter but still some efficiency is lost compared to the federal exhaust manifold which output goes directly into the turbo. I personally do not think it is worth the money at this point. I get 24-25 mpg consistently on mine but that is about 50% stop and go and the rest cruising at 75 mph on the highway. The last couple of years I have put very little money into maintaining it and it runs very well. I think the best improvement (few more mpg) was getting some rebuild injectors with new nozzles since mine were the originals with 300K.

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85 300D Turbo

x2 if you have 100,000 miles or more on your injectors, absolutely have them rebuilt with new nozzles and you'll see a boost in mpg's.

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'98 E300 turbodiesel