the additional 8cc going from 66mm to 68mm will not add much power.
what it will do is make that cylinder junk the next time it needs rebuilt as you will now be out of bores.
only bore as much needed to restore the cylinder to fit the next size piston.
do not waste bores trying to find power, those few cc's are not where it's at.
and yes, you need a reputable shop to do it.