I have a B50-11L comming and it looks like it is going to be a hell of a motor for the Eco 8. I am really going to let it rip with this motor installed.
I know from other sites that the B50-14L is a good all around(basic flight and acro) motor.
Im pretty sure that the B50-11L to 15L are all very good motors for the Eco 8(which one depends on pack used and pinion). Also, the B50-18S is a good motor for the Eco 8 according to Kontronik and other who use the motor. I have read that the B50-14S-19S are good motors for the Eco 8(which one depends on pack used and pinion).
Personally I think the B40-xx are too small for the Eco 8.
Here is a lot of motor test data in various helicopters including the Eco 8. It is definitally worth looking over.