Daft question, but have you checked your tyre pressures recently ? I've you got low pressures anywhere, that can have a serious effect on MPG.
The benefit of knowing an enthusiastic (& hopefully knowledgeable) owner with vag-com is that you have time on your hands, and you can log the outputs of numerous sensors to try and spot a
problem. For example, fuel & coolant temps, airflow, boost pressure etc.
VAG-COM (or VCDS as it's now called) provides you with all the same facilities as the computers that Seat will use, however I suspect all they'll do is plug in and look for fault codes, and that's it - if none are found, there's nothing wrong.... Some problems aren't "severe" enough, in the ECU's eyes anyway, to flag a fault - yet can cause problems with running. That's when you need someone who knows what they are doing to investigate.