Thanks for all the replies. At first I was thinking that I could accomodate the large grain bill by simply doing a thicker mash (1qt/lb or less?) but then conversion comes into play, and got me worrying that I wouldn't get full conversion.
If NB says that it maxes out at 15lbs, then I should, in theory, be able to mash around 13lbs.....albeit in a very confined space. Just gotta make sure I hit the strike temp...and keep it there.
Anybody have any good clone recipies (IPA's) for a larger grain bill that has worked for them in a 5gal MLT?