Misfiring isn't only from increased pressure in the cylinder, it is caused by excessive amounts of fuel also. If they are running good AFR's in the top end and cylinder pressure is under control they should be able to run 1.1mm just fine, if not the coil packs may be flogged.
You are right, plugs do wear slowly. If they are wearing fast you may be overheating them, perhaps try a BPR7.