If the geometry and total gauge of the two are not that dissimilar, then you have your results - more the same than different. You will not get a sea change in performance from break in. Your ears may acclimate to the sound, but that should not be confused with performance improvement.
Sounds like the upgraded Cobra fails the Law of Efficacy in comparison to the Topaz, in that there was not a significant, easily discernible difference (we could say "worthwhile", as opposed to your experience, "hardly any difference at all"). I would suggest going to an entirely different line of cables, where you likely would hear a much more distinct difference.
There is one other possibility, which we could eliminate if we knew what equipment you are running. If you have closer to Mid-Fi gear then you likely would not be able to discern differences between the cables nearly as easily as if you have higher end gear. This is not a judgment, merely a statement of fact. Rules of thumb are not exact, but can be helpful. If you have any or all of the following, 2 way smaller monitor speakers, vintage gear, or a rig not much beyond $5K, then you will likely not hear much difference. If you run a rig with what is approaching a true full range floor standing speaker, current and higher end gear, maybe in the $30K-$40K total cost category, then you should be able to hear more of a difference. IF you have that kind of rig and the difference is marginal, then the cable is likely a virtual clone of the earlier model, or Audioquest made a lateral move (at least to you) in improving their cables.
Even when I had a modest rig I could hear what I felt at the time were significant improvements between AQ cables. However, that was within the same line; the Topaz is the old line, and the Cobra is the new. What differences exist between them, aside from cosmetics, I am unsure. A breakdown of the configuration of the two cables might provide the answer.
I never depend on break in period to determine whether a change is efficacious. If the difference is not significant immediately, discernible within seconds to a minute or two - several minutes at the most, then it was a mistake or mismatch and if kept will limit how much improvement you will achieve, as you will be settling for only a marginal change. Any change which does not strike you as impressive is not worth considering. There are FAR too many fabulous improvements out there in cables, components and speakers to settle for mediocre changes.
