Even modifying the receive antenna on a part15 devices is technically illegal, mind you only technically, I severely doubt the FCC will nocking your door down if you do it.. Personally as long as you aren't actually increasing transmit power puting high gain antennas on a wifi rig is safe common practice. Most people wonder why they're so uptight, the simple answer is these are the only airwaves we have, if the rules are tightly controlled and regulated they're ruined for everyone. All it takes is one poorly built home transmitter to ruin the airwaves of potentially thousands of people in a large city. It's also not exactly healthy to be submitting oneself to ever increasing concentrations RF energy just cause someone wants wireless access a few hundred feet further.