Downpipes: what they really give, and what about the catalyst
Numbers without rounding in our favour: what changes on the stock map, what changes after calibration, and the trade-offs.
The downpipe is the first section of pipe after the turbocharger. The factory part almost always contains a catalyst and a restriction, and that is where the engine loses most at high rpm.
What it gives on its own
On the stock map: between 10 and 25 hp depending on the engine. That is the honest range, and it is small. The real effect is elsewhere — back pressure and exhaust gas temperature both drop.
Why a before/after dyno misleads
A modern engine adapts: on the stock map it simply uses the same air slightly differently. The real gain appears after calibration, where the downpipe contributes 40–60 hp as part of a package.
About the catalyst
A straight-through downpipe means no catalyst. That affects roadworthiness testing and emissions rules in a number of countries. It is the owner's decision — and better made before the purchase than after.