The blacks are still sligthly better in the z550a,sony image processing varies even though they may be advertized as BE3, i have said this before, a perfect example is t550,bx300 vs s550,ex400 respectively.
The 200hz does make a difference sure it may not be by much.
The z and x series are sony high end to flagship models,they have different backlight,really good lcd panels,with sony specific color filter,outside glass/plastic layers etc.
Again the z450 which has the be2 vs w550a when playing sd content,the amount of shadow details the z450 shows is more then the w550 which has the be3