After much reading, I came across a thread about the Teves Mk20 ABS-ESP which mentioned that the valves leaked prematurely and this resulted in a low pedal to varying degrees. The valves should not leak and provide high pressure fluid in front of the MC piston to stop it going to far forward. This leakage could explain why some people have less of a pedal drop.
I would replace the existing unit if I could assured of being able to bleed it.