Just for posterity... the 10mm and 12.3mm keihins can be 'safely' bored out to 13.5. More than that and it goes through the wall of the venturi out into the top of the carb. JB weld (ghetto, I know) nicely builds up the outside 'waist' area of the venturi and it can be then bored out to a 14mm.
Any larger than a 14mm, and the idle air tube thingy gets in the way... larger than 15mm, the emulsion tube area gets breached.
So, there we go.
Anyone thinking about making a 14mm, note that the throttle butterfly valve rod needs to be slimmed down because the cross sectional area (bore minus the valve obstruction at wide open throttle) is equivalent to something like an open 13.6mm bore.
Just buying a bigger carb is a good idea, too. But, hey... science, right?