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If anything, you will run richer with a bigger downpipe. The way your emissions system works is by measuring the gas density in the exhaust with the O2 sensors. When you have a larger pipe, there is less density of gas with the same amount of air being ingested, which is detected by the car as running rich, therefore the car will have a tendency to run rich. So if you are running rich now, this won't help. It wouldn't be that bad of an idea to run a boost controller (the kind that works with the bypass) just to squeeze out 1-2psi to lean things out a bit to give you a lot more power and more fuel mileage. Just don't go too far tripping an overboost situation. Also, I doubt you were ever getting 15-17 psi. The stock wastegate won't sustain such a high boost. It begins to leak at just under 15psi. So unless you have upgraded your wastegate, I would double check your boost meter, 'cause 15-17 psi is a theoretical impossibility.
I have never heard of this v-speed you speak of, so I have nothing to comment on that. But from what you describe, it sounds like another one of those too good to be true things again.
As for your mileage problems, there are so many possibilities, but I am willing to bet it has something to do with the ecu upgrade.
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