That's an interesting question, and I'm not sure. My gut instinct is to say yes, the same wort darkening occurs in an AG wort as in an extract wort, but I can't say for sure. I'll do a little research n report back.
5 min to end of boil is a good time to add extract, because you need to stir it in well and if you add at flameout you risk it dropping below pasteurizing temp before its all stirred in. 10 min will likely make more of a color difference than you might think.
If you can't seem to wrangle the color problem, you might consider tweaking the recipe to use more pale and less amber extract. It will change the flavor a little, but if your using specialty grains (which I HIGHLY recommend... Definatly one of the easiest ways to improve your all extract beer) then this will allow you a lot more control over the flavor and color, and you could use all pale extract and tweak the grains ratio to get the exact color you want.
Above all experiment, that's the beauty of home brewing!