I measured the power with an Emeter. It's a tool which plugs into the battery side of the ESC and then you plug the battery into the Emeter. I adjusted the varioprop pitch on each prop so that each motor was drawing 20 amps at full throttle. The voltage ended up being about 11 volts for about 220 watts. The entire setup drops the voltage on the three batteries a little more to about 10.8 for a total max wattage of 860 watts. That's definitely more power than I need. The model flys like a big fighter and will roll and loop at much less than full throttle. Still, the power is nice because the model will stall easily due to the relatively high wing loading of around 30oz/sqft. I believe the weight of mine isn't quite 8lbs. It's probably closer to 7.5lbs.

I'd be amazed if the 96" B-17 could come out at less than 15lbs RTF. That means at least 1500 watts for sport performance. I'd imagine even that weight would require keeping the tail very light and moving everything as far forward as possible to reduce the weight required to balance. I guess if the model is not sheeted in the tail it would be possible to come in at less than 15 lbs. My plane is definitely heavier than it could be. The fuselage is fiberglass and the wings and tail are balsa sheeted and finished with polyurethane and primer.

Here's my build thread if you need any more info:
http://www.rcgroups.com/forums/showthread.php?t=536562