I think that it could be a couple of different issues. King bearings have a higher amount of drag in just about all of their components to the the amount of grease they put in from the factory. This is part of their secret to their stuff lasting for decades. Also it can also be due to the inherent design of a GXP. It is a stepped spindle. That being said, it should spin a bit more freely than 2 revolutions with minimal torque.
First off, I would take everything apart back to the frame and start from scratch. You should be using NO BB cup spacers (the black 2.5 mm spacers) since it is a 68 mm BB shell. The BB cups should be torqued down to around 40 nm. There should also be no spindle spacers with the use of any road double crankset. Make sure you put that compression washer (spring ring looking thing) on the drive side of the GXP spindle before you put it through the BB; a little lube won't hurt it too. Grease up the contact points of the spindle, put it through the BB, and put the other metal ring that fits on the GXP tip in place. Lastly, (I think this is the part that could be affecting your drag) make sure to put that silver adaptor washer on the spindle before you put the crank arm on. It would give you a false torque reading without this washer. It could also be due to any BB cup spacers. But definitely, definitely, torque it down the the correct settings. (45 nm). The whole GXP system is particularly contingent upon correct torque. Also king makes two different types of road bottom brackets: one for GXP with a stepped spindle and the other for shimano. Make sure you have the stepped one. King makes very precise stuff, but it can be complicated with all of their different designs to put it together.
Now once everything is in their perfectly, and you spin it freely, it will still have a bit more drag than that of a shimano DA (Both broken in). That is definitely negligible, and if anything, take comfort in the fact that that bottom bracket will likely last much longer than the shimano dura ace (King makes a grease injector tool too shoot fresh grease in too).
Once you know for sure that everything is in there correctly and torqued down to the correct spec, I would just ride the hell out of it and shoot some grease in their on a biannual basis. You could always put some lighter, less viscous grease through that can help to reduce drag.
I don't know if you were in town for aggie muster yesterday, but I work at BCS bikes if you had your bike in town today. I hope this helps.
FTA C/O 2011