I did not fly sparge properly, undershooting the preboil gravity by quite a bit, and ended up using more sparge water to extact sugars. I then reduced away most of the additional water, and ended up with an OG of about '87 (with a somewhat bigger volume). The target was 1.1. I'm thinking about adding sugar, honey or similar to the primary fermentation (going on as we speak) to get it up to or closer to this level.
My question is fairly general: What might I expect from doing this? From what I understand, I'll need to add almost a kilo of, say, honey to achieve this. BeerSmith warns me that this will lover the estimated FG from '18 to '15. Does this seem reasonable? One of my main reasons for considering this is I still used the same amount of hops, and having first worted quite a bit of it I'm a bit concerned about balance (with the greatly increased boil time). Furthermore, I want the brew to be heavy for the winter months.
The relevant recipe is the following but with more hops, Target substituting Admiral, and English Dry Ale substituting Irish Dry Stout: http://beersmithrecipes.com/viewrecipe/62000/imperial-stout