Sugar replacing some malt in RIS?

Long time lurker, first post. In honor of the olympics in sochi, I am planning to brew 10 gallons of the below this weekend:

https://www.homebrewtalk.com/f68/russian-imperial-stout-2011-hbt-competition-category-winner-238807/

I mash in my 16 gal Bayou Classic kettle, drain to buckets, clean the kettle, dump the buckets back in to boil. I have some smaller turkey fryer pots I heat up sparge water on a separate burner, and batch sparge.

I don't think I can quite fit 40+ lb of grain and enough water comfortably in my kettle. So I was thinking to mash with maybe 5lbs less base malt, and add some form of sugar to the last 15 mins of the boil. I tend to like my beers a little dryer anyway. What type of sugar do you think would be best to complement the recipe? And how much sugar per lb of base malt replaced to put in? I have never used extract before other than making starters, maybe should just buy some extract instead?

1) Molasses
2) Dextrose
3) Honey
4) Table sugar
5) Turbinado
6) Extract
7) Other?

Why not use extract instead of sugar? That would be the best way to go, I think.

But, if you want to use sugar, I'd go with some form of cane sugar that still has flavor to it (turbinado, demerara, sucanat, whatever). You might want to cut back a bit on the bittering charge if you go this route. How much depends upon your efficiency, but a good starting point is subbing 5lbs malt with 3lbs sugar.

I agree with GuldTuborg above in all respects. Extract is good if you want maintain the profile of the original beer. But sugar is fine in very strong beers, too.

For a very dark beer like an RIS, you are going to lose the character of most forms of more mild sugar. So either go for plain ole white sugar, or do something adventurous, like mixing in a little molasses. This is one of the rare situations where even the darkest stuff (blackstrap) could be appropriate, though you might not want to use too much of it.

Extract may be your best option, aside from brewing 8 gallons instead of 10. I'd buy more than you think you'll need, given the strong possibility that your efficiency for this beer will be prety low. If you use table sugar, I'd increase the amount of Carapils. Another option is to drop the Special B and use some Dark Candi Syrup, although 3 lbs of it would produce a very different beer.

Thanks for the replies, so I brewed it yesterday with recipe below.

28lb Maris Otter
3lb Roasted Barley
2lb Special B
1.5lb Chocolate Malt
1 lb Carapils
Mashed starting at 153->temp loss to 148 over 60 mins (mash in un-insulated kettle, this is pretty much how all my mashes been going this winter)

Any comments how the temp loss impacts the sugar profile? Based of other threads, I would think not much since the conversion should happen quickly

Mashed in 10 gals
Batch sparged with 6gals after filling one bucket

Added this in with 15 mins left in boil (disolved in a separate pot two quarts of water)
12oz Grandmas robust molasses
2lb Turbinado sugar
1lb Honey

Hops
1.5oz Magnum @ 60min
4oz EKG @ 30

Got 12.5 gals @ 1.085 (did not calc effiency but I imagine ~70%?)
tasted great

Split it between 3 buckets, and split a 05 yeast cake of a porter I just kegged between the buckets

Its 58 deg F in my basement and all three buckets have action this AM, and smells amazing in the room. I'll post how it came out this fall.