I'm wanting to brew a dry stout, but I am trying to save myself some time, so I am using extract, and ran across some discussions on cold steeping specialty malts. I'm fairly certain that a ~24h cold steep will extract what I want from the toasted and roasted malts, but I also want some flaked barley in this wort. Will a cold steep extract the proteins and such that flaked barley brings to the party (for mouthfeel and head retention), or does it need the higher temperature steep?
Nope. Flaked barley needs to be mashed. You'll get nothing but starch from it in your beer. Not only do you need a higher temp, you need some base malt in there an you need to do a partial/mini mash at least.
If it's the head retention qualities of the flaked barley you are after, you could add 200g/0.45lb of carapils per 19l/5 gal. It will add head retention, and hardly adds any color, and such a small amount will not affect mouthfeel.
I usually brew my stouts with an Irish liquid yeast, but I've also used US-05 and then they come out really dry! If you use US-05, it's probably a good idea to double the carapils, to round out the beer a little. If you're used to drinking Stout on Nitrogen, but are bottling or serving on CO2, you'll loose the thicker mouthfeel that serving on N2 gives. You can add 300-400g/0.6-0.9# carapils can also help create a slicker mouthfeel.
Unmalted cereals always need mashing, since they contain no diastatic power, and can't convert the starches into simple sugars. If you steep, you'll extract some beta-glucans that give some of the mouthfeel, but you'll extract far more starch. In a light beer, it will be clearly visible. In a stout, well, it's not going to be all that visible - so you might try it. Oats are particularly good, and add a lot of smoothness even in small quantities (<5%). For the best beer, you should really mash, but if saving time is more important than getting the absolute best beer you can, then add a few percent of oats and the carapils and you should get a nice mouthfeel with a good head, and at the most a touch of cloudiness.