DGHOB:
We usually consider the amount of water vapour( hence the contrail) indicative of the amount of combustion going on in the engine.
While I realize that it is related to the relative humidity of the air at altitude, it is still valid when comparing the amount of water vapour issuing from our engines vs. the -400's, when we're both in the same airmass.
It became apparent, right on my first takeoff and climb in the 340, that Airbus uses the reverse philosophy of Boeing, i.e. just enough thrust to get off the ground and lots of wing for cruising, while Boeing uses lots of thrust and just enough wing, at least in some of their designs. They may have been designed differently, but that is sure how they seem to fly.
The last chapter in the 340 AOM has some propaganda comparing the lifting capability of the 400 vs. the 340. While the 400 can lift more, and lift it to altitude quicker, once the 340 gets up to altitude, the much-lower fuelflow more than makes up for the sluggish climb.