It's probably the Hirsch upgrade, if it was done under the warranty. A dealer can remove it. We're any other mods done? Springs? Grille? Trim?
If it is the Saab approved Hirsch upgrade, it will be efficient, so I doubt you will improve the fuel consumption. If you don't use the extra performance you won't be going to the thirsty upper reaches of that performance. So going back to standard could remove he efficiency and fuel consumption might actually be worse. The 2.8 is a thirsty beast. Each one has its own hole in the ozone layer!