Originally Posted by Cugel
You're correct - the aroma and flavor hops will be greater, and you'll get less than expected bittering contributions.
Hop utilization/extraction decreases as wort thickens, and vice versa. So what I do is boil a thin wort (typically 1 lb DME) with the full complement of bittering hops that will give me the IBUs (which is less than I would need if I used a full - thick - boil) I want and then use the regular amount of aroma and flavor hops. Then add in DME at the end of boil, before cooling. Works fine for me.
Note that this is a byproduct of the bittering estimation formula that you use. If you use Rager, you won't see the same thing (not to the same degree, anyway, since it's gravity correction is a lot more restrained).
None of those bittering formulas really account properly for why many higher-gravity beers get less final bittering from hops than many lower-gravity beers do.
The most recent research seems to indicate that wort gravity does _not_ affect how much alpha acid extraction you get during a boil. However, break material takes out some acids with it, so most higher gravity worts will wind up with less bittering in the end--but the effect of gravity in extract brews will be a lot lower than in all-grain (since there's correspondingly less break per gravity point), and you shouldn't include simple sugars in the gravity calculation for purposes of bittering estimates (and wheat/rye and other things that generate a lot of extra break material will have a greater affect than the simple gravity calculation would have you believe).
Whether or not you buy that, late addition of most of the DME is still a good idea to avoid Maillard reactions affecting the taste and color of the brew.
On deck: Little Bo Pils, Bretta Off Dead (Brett pale)
Secondary: Oude Bruin, Red Sky at Morning (Sour brown ale)
On tap: Saison Duphunk (sour), Amarillo Slim (IPA), Earl White (ginger/bergamot wit)
Bottled: Number 8 (Belgian Strong Dark Ale), Eternale (Barleywine), Ancho Villa (Ancho/pasilla/chocolate/cinnamon RIS), Oak smoked porter (1/2 maple bourbon oaked, 1/2 apple brandy oaked)