Not sure on the older R6's but the 03 and 04's can't go much, if any more than 2mm overbore. Which still is quite a bit.
Can't remember his name, but a guy on messagenet did a 2mm OB, and lots of other shit. I'm gonna look into it when time comes to rebuild.
the problem with boring the new 6's is the cylinders is that they are Nikasil lined. So you'd have to send them off, have a replating shop do the work. And from all my experience of blowing up and rebuilding two-strokes with niked cylinders, I know it's not cheap where i'd get it done.
Nikasil lined cylinders have way tighter tolerances which is the reason for allowing plenty of engine warm up, cause cold siezure comes easy! And once these cylinders are punched so far, theirs no fixing them, or replacing sleeves etc..