Blindly boosting signals (especially above spec) lead to problems, and this looks like one of them. Try turning the router back to normal levels and see if the problem goes away. Re aiming the antennas may also help.

Modern Macbooks all have 802.11n cards and antennas, and something with the MIMO part of the radio may be confused sitting near to the non MIMO router. Unfortunately I don't understand wireless a ton to really explain this well.

*edit* I also have to say, I'm quite surprised that you are putting 3rd party firmware on customers equipment. Beyond boosting the radio power beyond spec, what advantages does this serve, vs the possibly increased support cost?

Heres an article talking about what increasing that value does. It also increases noise in the signal, and it may be pushed beyond spec to the point where the Macbook can't deal with it at close range.