As Dave said: std gap 0.7mm (and do not gap the iridium ones as it kills them)
0.6, 0.65, 0.7mm ... All are common gap sizes for forced induction applications, and realistically very little in it and you would not notice any significant difference within that range.
0.9-1.1mm gaps are more common for NA cars, not forced induction... which is indeed because under boost conditions it will take more energy for the spark to jump the gap.
@Zeiko: if you have new plugs typically the gaps quite accurate out of the box. You checked and saw it was somewhere around the 0.65-0.7mm mark... which is fine. If you still have the issue after fitting new plugs with gaps like that then Dec is spot on and sounds like one or more coilpacks are getting weak --- and / or bad earthing can also be a (contributing) factor (these are not new cars and earthing can get corroded which can also affect spark). Whether coilpacks or earthing... it would be more evident at higher revs and/or boost as there is more resistance for the spark to jump the gap, which takes more energy. I'd get the coilpacks checked / have new ones fitted... and clean up the earth strap connections on either end as part of the job. As for not showing error codes: only if one or several totally fail and the issue is there for a prolonged period would you expect to see a diagnostic thrown up... but if only happens for a few seconds during driving (as you'd back off when it happens) it wouldn't typically throw a code immediately.