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If the car has no rear sway bar, it does not have the sport suspension.
Is it worth adding one? It depends on what your intended usage is, of course - and also, I guess, what diameter bar you intend to add. I forget what the stock sway bar is, but I think it's fairly wimpy - maybe 12mm? But I guess that's better than nothing; I've never driven a 128i with the stock suspension.
That said, if you're not going to upgrade the non-sport springs and dampers, I would absolutely leave the sway bars alone. What's the point, really? Sway bars are kind of a finishing touch, and it makes no sense to go to sportier sway bars when the foundation of your suspension is stock.
However, if you're going to upgrade the rest, then yes, add the sway bar. As you say, it is a bit of a pain in the ass - the rear subframe needs to be dropped a bit, so make sure you do the RSFBs at the same time (these will have a much more noticeable effect, anyway).
I wouldn't go with a larger than stock rear bar unless you're going to do more serious suspension work, as well as an LSD. I've got a 20mm rear H&R bar, but I've also got an LSD and 700# springs in back. They all work together. The car definitely corners flatter, but without the LSD I'd be losing a lot of thrust on sharp turns.
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