AGU can upgrade to Calibra turbo yellows or Saab reds with a VR6 MAF
All others can upgrade to the TT injectors with the MAF from the same car. You will only need injectors/MAF if you want 230bhp+ and you will need all the other supporting mods to go with it
Also bear in mind injectors aren't something you can just fit and drive away, you'll need a custom map too