I just brewed a 5 gallon batch of this today. This was my first endeavor into high gravity brewing.
Filled my 10 gallon MLT to the brim with the mash. I ended up mashing at 150. I double batch sparged and had about 8 gallons to boil. I boiled for an extra hour for evaporation, but it wasn't enough. I was shooting for 5 gallons of 1.128 OG wort and ended up with 5.75 gallons at 1.108. So, my efficiency (72.5%) was pretty close to my target (75%) according to BeerSmith, but I needed to boil for longer.
I'm using WLP99 Super High Gravity yeast, but I guess that'll be overkill now that my gravity will only get me to around 11.25% alcohol if I'm lucky.
Anyway, I just thought I'd share. Today's lesson: Get some volume markings on the brew kettle. I'll let everyone know how it turns out in around a year or so.