Yea, you'll have a higher gravity. If you use any brewing software, I think they all have a recipe scaling function. Hope your grain isn't all milled together.
If I were in this situation, I'd either do the 7g boil if equipment allowed, or just have a higher gravity version of the beer I intended. I suppose you could also just dilute at the end.
FYI the BCS recipes are all scaled (except where noted) for 7g pre-boil, 5.5g collected into the fermenter, and 5g into the keg/bottles.
Elkins Creek Brewing Company
Secondary: Bavarian Hefewizen (extract)
Bottle: Blue Moon clone, Sam Adams Boston Ale Clone, Christmas Chocolate Stout (All Grain)
Keg: AHS West Coast Pale Ale, BierMuncher's Centennial Blonde (All Grain)