There is of course a point beyond which adding more sail is unlikely to provide a higher percentage of planning on most boards.....I say most, because the wind/sail range of most slalom boards is huge and that ability is now being replicated with the newer freemove boards. The biggest and smallest useable sails may not be optimum but will work.
I think the issue here has to do with the very variable windstrength typical of low pressure driven systems. The question is always what board/sail combo will deal with both the gusts and the lulls. Given the excellent top end now provided by most sails, there is little or no penalty in choosing a larger sail and that may well encourage some to go as far as overloading any particular board. It is also the case that some larger boards can now deal with heavier chop etc.
I select board/sail combo that ensures I will be planing all the time.......sometimes the best way to ensure that is to use a relatively small sail on a bigger board, with a smaller board on standby for a quick swop if the wind fills in. Rather than "overload", the thing I notice most is people using boards that are just too small for the conditions and their weight..........presumably for fear that a bigger board will not handle the gusts or water state. Constantly coaxing a board up onto the plane with extended periods of slogging is not only no fun but flipping hard work!