I did 15lbs once - OG was 1.077, eff was 81% (added 1 lb. candy sugar to wort). The mash was really thick 1qt/lb. The 5gal igloo was filled to about 1 inch below the top. I think I would not do this again, although I am enjoying the brew.
Yep, 14 lbs. is the most I successfully fit in my 5 gal. I tried 15 lbs and it was a mess.
Now, of course I have a 10 gallon cooler so I can have more range in gravity and room to add infusions.