Efficient dynamics is a bunch of stuff like an electrical motor driven AC compressor to improve gas mileage.
The other main 09 M5 differences are new mirror design and hard disk based nav.
I am looking at an 08 M5 demo with very, very low mileage.
I would be very careful of a used M5. I would definitely search M5Board to (as the previous poster suggested) what has been done TO the car.