You'll have concentration of the existing alpha acids, so it will be roughly 20% more bitter than at 6 gal. I suspect you'll also get increased utilization, but even going from 60 minutes to 120, for example, isn't a huge increase. If there were large late hop additions, that might be different. A 20 minute addition would effectively become an 80 minute, and so on.
Along with color, you'll also get increased production of Maillard products, and I'm sure that will have perceptible flavor effects.