Yes, it's possible to bore them out. But, it would only useful on an engine designed for racing, in which case it would be wiser to get a larger carb anyway. The venturi accelerates the air at it's narrowest point, when it widens again this creates the vacuum which sucks the fuel out of the nozzle. So, boring out the venturi will stop the carb working at lower revs; not much use for the applications type 3 carbs would be used for (i.e. street,) is it?