I've run the NGK Iridium BKR-6EIX's , 7EIX's, and now 8EIX's.
The 8's I'm running now (coldest) don't seem too good when the car is cold. Doesn't like starting, and idle is lumpy. However once the car is up to operating temperatures it seems much better at the top end.
The 7's didn't seem to make a massive difference from the 6's TBH.
I think my issues are with my OEM cat and DP, and I've just been trying to fiddle with other stuff to alay the issue of replacing them.