"How much engine braking" is a difficult one to answer - lots!!
The slipper clutch is designed not to lock up the back wheel when you've changed down through the gears.
So in theory you'll get maximum engine braking without locking up the wheels. As I've been having false neutral issues anyway, I've found myself going down a lot more than normal without meaning to (in my anxiety to get any gear!) and found that the clutch just releases (slips) as much as it can without locking the wheel up. (am I repeating myself)
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As for mileage, no reason why it shouldn't last a long time, unless your racing every weekend! - So depends upon use really
Tim