It seems like they are chasing their own tails and replacing things for the sake of it. What are/were the codes? That is the only to tell what is going on. I would ask for a full report on what they changed and what 'code' or otherwise justified replacing that particular part. This is why I never go to a main dealers. Useless.

 

I have a 2014 gti prestige.. done 142k miles now owned since new.
I had water pump replaced under warranty along with 2 high pressure fuel pumps, front shocks etc etc.
I also had a cylinder head replaced under an independent warranty.. yep £4.2k
Something about a fixed cog that is a known fault which wears and throws up a fault code. They can't replace just the cog but whole cylinder head.
I don't remember misfiring but a little Jerkey on acceleration whilst code came on..
Buy a £4 diagnosis module from ebay which you plug into you odbc port.. download app on your phone and allows you to check codes and fault diagnostics.
I good luck