I brewed an imperial stout yesterday. Last time I did one I had horrendous efficiency and ended up around 8.5%. I wanted to make sure that I got higher this time so decided to do a reiterated mash. I split the grain bill in half and did two mashes, First mash had a sparge, second was no-sparge using the wort collected from the first. Ingredients were:
9.4 kg Maris Otter
0.8 kg Brown Malt
0.6 kg Amber (Crisp)
0.4 kg Black Malt
0.4 kg Crystal
0.4 kg Roasted Barley
1 kg Invert #3
0.5 kg table sugar
104 g Green Bullet (11.2%) - 60 mins
39 g Northern Brewer (7%) - 60 mins
28 g Magnum (9.2%) - 60 mins
Both mashes were for an hour at 65C. Once cooled, I aerated the wort for at least 30 mins with a pump and then pitched 2 packs of Windsor and 2 packs of US-05.
All went pretty well. In fact too well! I ended up with 21L of 1.129 wort!

Its bubbling away happily now, but I'm worried about whether it'll finish out ok. I'm hoping I can get it down into the high 1.020s. If so it'll be around 13.5%! I'm wondering if the yeast will cope with that or if I need to get my hands on a high gravity yeast that I can add to finish it off.
Any of you had experience of brewing a beer like this?