Once you've had mis-fires the plugs foul very quickly (Optimax a specialty), make sure that after you've gapped them, you clean them prior to installation.
I doubt that the spark gap is hurting you - 1.1mm is OK for the lowish boost pressures that you are experiencing problems at.
Anytime I hear of spark break down on a turbo engine (given that plug leads and plugs are OK - and I would buy enough plugs to do 2-3 changes) I ALWAYS suspect the coils.
If it were happening at higher revs and boost pressures, then I would look elsewhere.

What coil(s) is it running?