Does any one here who has increased their boost run colder spark plugs (Steve Webb I think has mentioned this in the past)

Part of an odd running problem whilst on the motorway is that I put my foot down at about 60mph and the boost goes straight up to just over 1 bar (manual boost controller) and you would expect quite a good turn of speed, but instead there is a smaller amount than usual, a bit more noise and its hesitating to pull properly.

So, I suspect its the plugs not being able to cope with the boost which is no longer ECU controlled...

Any thoughts, and if anyone has changed for colder plugs please could you post the part numbers?

Thanks
Jack