If it doesn't need it I see no reason to bore it out 1mm. Power gains will be minimal, you'll need to grind the powervalve for clearance, and port timing may be slightly altered as the ports do not exit the cylinder at perfect 90 degree angles. Plus if you screw it up later it can just be bored and replated, as opposed to welding then boring and replating, which I am sure costs more.
If you want a little more boost just have it ported. I run FRP porting, head mod, carb bore, FMF rev pipe/Turbine Core II and Boyseen or FMF reeds (feel the same to me). It rips, and it's stock bore.