Can't see that a 3% increase will noticeably affect performance; that's ~1/2" on the beam at the widest point and ~6" on length. Have a friend who has enlarged boats (OI, Petrel) by 7-8%, running forms through a photocopier, then adjusting the spacing between forms, and his boats perform well... wouldn't go much beyond that, however, for all the reasons stated.

It's easy to add height to the forward part of the cockpit to allow for larger feet. At Jay Babina's suggestion, I did so with my OI, cutting the templates horizontally for the four stations forward of the thigh braces, then added progressively 1/8", 1/4", 3/8", 1/2" to each one, creating a gradual increase above the sheer line. The bend back to the rear deck was a little sharper, but with a little steam and coaxing, it came out beautifully. Hard to tell the boat from one built to spec, but it's a little more comfortable paddling and getting in and out.