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.
Relax, don't worry etc. and so on.
Primaries: Old Ale, Barleywine, ESB, Scottish 80/.
Secondaries: Lime Wine, Strawberry-Banana Mead, Carmenere (from 144 lbs of grapes!), Engl. Barleywine, Modded JOAM, Concord Grape Pyment.
Kegs: Choc/Coffee Stout, Saison, Dry Stout.
Bottles: Belgian Str. Dark, Dubbel, Cider X 2, Modded JOAM, RJS Pinot Noir, RJS Aussie Cab. Sauv.
Coming soon: Blueberry Mead.