Good point re. running the fuel low, I had an identical problem with an old MGB, so I know exactly what you mean. I hadn't thought to be careful about it with the Scirocco being a newer car, but you're probably right.
In contrary to that, I did run down my last tank and only got 300 miles. That's an improvement on what I was getting before, but I used 52 liters of the cars 55 capacity, which is about 26mpg. It's running perfectly in all other respects now, so its a bit confusing. I haven't been driving it hard, I've done nothing but my commute on that tank, which is a 20 mile stretch of motorway so would expect more like 36 tbh.
I'll do what you said and mark the dissy position with tipex and try putting it forward a bit, though I'm not sure that'll give me quite mpg boost I'm looking for.
This cam belt change has caused such a huge ball-ache, my car was fine before all of this! haha