Two 6870's would do perfectly fine, and be the best bang for the buck, because you already have the first 6870. Will they be the fastest? No, but are you going to get the performance possible from a 7970, or even a 7950 with your 1055t, probably not.
Micro-stuttering is an unfortunate by-product of multi-GPU configurations. The actual seriousness of the issue is very subjective. Both the AMD and NVidia platforms experience it, but my understanding is NVidia's SLI doesn't exhibit it as noticeably. Either way, I doubt there would be as many multi-GPU configurations if the solution was totally intolerable.
As far as bottlenecking - there will always be a bottleneck. Whether it's the CPU or GPU, or something else, is up to the parts you choose, and the application.