That said, you probably can get by with a bit less in some areas. Feathered Friends' Icefall (around $400; www.featheredfriends.com) is designed for expedition use, meaning it could handle Denali and Aconcagua. I'd agree that you'd get more all-around use from this piece, although it's still too warm for just about anything in the United States aside from winter climbing and camping in places such as Wyoming. I think you'd do fine with this piece.
Really, same for the Widgeon ($500). Although it's rated to minus ten degrees, Feathered Friends is typically pretty conservative in its ratings. You could wear the Icefall to bed on cold nights, along with some heavy fleece pants, and be warm enough. You can also add a vapor barrier liner ($25) to the bag, adding maybe five or ten degrees to the low end. Also, carrying an extra sleeping pada self-inflating pad plus a light, closed-cell foam pad such as a Cascade Designs Z-Rest ($36; www.cascadedesigns.com)will really help you stay warmer at night. You may still have a few chilly nights, but you're unlikely to freeze to death.
Good luck with your climbs! Big mountains, both.