Do the rear brakes need bleeding if you change the calipers? Sorry if that's a silly question, not had much experience with brakes..
Off topic but I've noticed im getting 2-4 mpg better since I changed both drive shafts a month ago....
Not sure about DSG related issues but what I've personally found is the 2.0 is nowhere near as lively as the 1.9 especially from standstill. We've had some fun before off the lights with me driving the altea and my girlfriend driving the 1.9 fabia and the fabia is gone before I've even started moving. It was the same with a Bora I had that had the 1.9 engine.
I think the 2.0 has a much smoother power delivery all in all. These are only my opinions though
I have a mk2 leon ref sport 140 bkd, and I too generally only get 420 til the light comes on and only 37 mpg. I had all the same concerns you do regarding economy. I have the dreaded siren noise from the turbo and think the bearings are worn so ultimately need a new one. Whilst waiting to do this I have had a terraclean done and only done one tank so far but gained an extra 42 miles from tank and mpg went up to 41.