Afaik, the ecu samples the fuel and changes various settings from that sample... I've noticed a power drop on one fillip of crap supermarket fuel.
PCV test, do a search, basically when car is running, open the bonnet and remove the dipstick for a second or two, don't leave it out!! If the car starts spluttering then your PCV is probably ok, if there is no discernible change, then you could need to change it.
VCDS is a diagnostics system, many members have it, check the VCDS in your area thread.
DV, can be checked from the engine bay on K04 but on normal GTIs youll need to the lift the car, best to just replace if the other elements have been ruled out, they are circa 30 quid, grab a revision G one.