as it turns out i bought the 8's but have never fitted them!
I got the car serviced by a VW dealer (major service) which included OE plugs
for what its worth, mine were 59,000 miles old and I was having P0030 Misfire Cyl3 fault codes on acceleration. These have now gone and my boost levels have increased
So old plugs had a really detrimental effect on my car.
For any GTI get new plugs regularly, the recommendation on here seems to be 7's for std cars and 8's for remapped.