I've never used the 100-400 and 1.4X with my 7D but I've had reasonable results with my 50D when using a tripod and Live View (Live AF or manual AF). I think if you really need that extra reach then tripod shooting is viable.
For hand held shooting I'd personally give it a miss on the 50D/7D and anything else not designed to work properly (i.e. AF) with that combination. After all, shooting at 560mm, hand held, when you're at f/8 maximum, and already into diffraction softening on the 7D, when you can't AF properly - just how much point is there to pushing your luck that far? With the trade offs between ISO, aperture and shutter speed I just can't see any value in it. Certainly I've stopped taping the pins in order to fool the camera. On a tripod, where you don't need fast shutter speeds, can keep the ISO low, and can AF accurately, if slowly, then I see some merit.
I get decent enough results when using the 100-400 + 1.4X with my 1D3, but again it's a combination I use sparingly, and I don't think I've used the combination for shooting any birdy action. Big, predictable bikes at Brands Hatch, yes, BIF, no.
Here's an unedited 100% crop from my 50D at 560mm, shot from a tripod with Live View and Live AF....