It's not all about the width...the shape matters too.
If you check some TGR threads about the issue, the general opinion there seems to be that over ~120-130mm in the middle and the traditional side cut won't work that well anymore...but there are still some shapes that make sense and ski well, like this:
Who knows where's the "reasonable" limit among the new shapes? Innovation with these new shapes seems just began.
Tip width is also something to consider: some say that w/ Pontoons your tips clap together way too much (and thus McConkey mounts them a bit "duck footed"?). Wayne - any problems with that?