well here was my experience -
holy crap that is alot of grain! I cut out 1 lb of maris otter and the 10 gallon round igloo mash tun seemed almost full before adding water! It was pretty amazing to see it suck up just under 7.5 gallons of water on top of that. I almost broke it up into 2 batches, but decided to go for it and am glad I did.
We were in my garage, and it was cold. We batch sparged. Added 173F strike water, hit like 155F. Was 153F at 30, gave a stir. Was 151F at 70 minutes, collected wort. It's real tough to get an accurate temp with that much grain by the way.
Anyways, collected 4+ gallons out of first runnings. Really let it sit to make sure we got it all. Added a little over 3 gallons at 189 and hit like 162F - should've used a calculator, with that much grain I guess you need to sparge hotter to hit 168F. Collected 3+ gallons, so ended with like 7 1/4 gallons. Pre-boil gravity: 1.080, thought I wanted to be like 1.092. But I think that's like 65% efficiency and I usually get 70% to 73%, got 80% once.
Nearly had a boil over in my 10 gallon pot - closest I've ever came. It was like actually boiled over but by some miracle defied gravity and held on while we stirred like banshees and got it under control.
My post-boil OG was 1.092, so a little low. Added 2 packs rehydrated US-05, I'm thinking with a lower OG I wiill have a lower final gravity, so it may be dryer/less sweet than it should be? This is definitely the biggest beer I've ever brewed and probably the limits of my system, if I were to add back the 1 lb maris otter. Smelled amazing! I plan to age this for 1 year.
What I'd change next brew - bigger mash paddle, shoot for 65% efficiency - add back the 1lb of maris otter, hotter sparge water, insulate cooler if outside in cold for the mash.