I have been brewing all-grain 11-gal wort yield-to-fermenters batches (10 gal finished beer) for the past +12 years, using a 15.5 gal keg kettle, basically via the Denny Conn Cheap n' Easy style batch sparge set up, and I do a minimum 60 minute boil. I plan to continue brewing with this set-up and boil length but I'm considering sourcing a slightly bigger kettle.
The reason is that experience has taught me that the more hot + cold break material plus pellet hop spooge I leave behind in the kettle, the less taste difference I get between the final two buckets of finished beer (which get racked into corny kegs), since I end up not having to strain out some of the spooge (through a sanitized fine mesh strainer) to get full volume (5.5 gal) into the second bucket when running off chilled wort from the kettle.
Again, in my experience it's not unusual to have to strain the last gallon or so of wort, even though I'm leaving 1 -1.5 gal of spooge + wort behind in the kettle.
Especially for hoppy beers I sometimes suspend a 5-gal paint strainer bag into the boil via a hop spider, and reduce my spooge load that way. Maybe I just need to quit being lazy and do that every brew.
Have some of you found it simplifies matters to brew a larger than needed kettle yield, and still get full 11-gal clear wort volume yield into the fermenters, simply by using a 17-gal or larger kettle, rather than a 15.5 gal kettle?