It all depends on your driving style for your MPG to be determined.
At first, you raise concern about why there isnt enough power in second and third gear, remember bro its a 1.0ltr car or so.
If you conclude my two above points, you can see that your driving style may have been aggressive because you feel theres a lack of power somewhere.
..and yes if you decat (remove the cat) there are extra power gains.
It might even be your engine, TRY HAVING A CYLINDER COMPRESSION TEST ..thats the best advice i can give you but who knows!
Good Luck,