There shouldn't be any reason to go to yet another dealer as if the seat shock was previously installed, the other dealer won't install yet another one because the "fix" via TSB was done.
As previously mentioned, one could also then replace the seat frame and/or tracs which will cure it for a while but eventually it too will get some movement again. All seats do have some movement noticed if you sit there and rock it.
I know, it's a newer car and still under warranty but one has to wonder just how annoying that really is? My '86 740 has had that problem since I bought it 8 yrs ago but I'm really only aware of it if I sit there and try rocking the seat. If I'm driving the car, I'm not aware of the seats movement at all.