AC3 and AC4 both have the same problem - they don't work properly with Crossfire. Beyond that, the limitations are the CPU, no matter how fast it is.
AC3 in Boston, in particular, can never be solved by any driver update, since it's a huge CPU bottleneck. I estimated that getting a solid 60fps in the CPU-bound areas would take something like an 8GHz i7 processor.
The same kind of CPU bottleneck exists in AC4, though it's less severe. Playing at 2560x1600 with a single 7970 (since CF doesn't work properly), I was GPU-bound far more than CPU-bound throughout the game. While the frame rate was occasionally 30's, it felt smooth overall.
This is with the 13.1 drivers.
You can't fix a game's design with new graphics drivers. All you can do is hope that the next title uses Mantle, which would probably eliminate most or all of the CPU bottleneck in titles like these (via better draw call throughput).