Gotta start at the beginning of the fuel injection equation with the intake side of things before we get to the exhaust. Intake manifold leaks aren't common like on 2/740's. With such severe performance loss, I'd look for a leak in an intercooler hose or the intercooler itself if its turbo charged. If not, like Klaus said, I'd be swapping an air mass meter. I'd go used original over something from a generic part store. New list price isn't even that horrible. They don't fail with such regularity that I'd be afraid of a used part.