I have done a 17.5 lb brew in my 5gal mash tun, but I had to use like a .8 ratio and it was super thick and sucked to work with. I also pulled off extra wort and boiled down to make up for the lower efficency.
I have done 17.5 pounds in my 5 gal. It was a super thick mash and I collected extra wort and boiled down to make up for my lower efficency. It was a pain but it worked, and was also my longest brew day ever.