As of now I would say that PileDriver will be approximately 10% faster than Phenom II / FX. I believe that is a reasonable assumption based on AMD's statement of a 5% - 15% increase in performance over prior generation CPUs.
In order for AMD to bridge the between their current CPUs and Intel's Ivy Bridge, PileDriver needs to increase performance by about 30%. If AMD can only manage a meager 5% increase in performance for each new architecture, then they are basically 5 generations behind Intel. This is a bit unrealistic. On the other hand expecting a 15% increase in performance with each new CPU architecture is a bit unrealistic too given AMD's more limited R&D budget and the fact that the PileDriver is is based on modular CPU packages where every two CPU core shares resources between each other. If this were true though, then PileDriver will be 1 generation behind Ivy Bridge.
A 10% improvement is a more conservative and reasonable estimate of performance increase for every new CPU architecture. If this is the case then when PileDriver is released, then AMD will only be 2 generations behind Ivy Bridge since the performance gap will be approximately 20%.
This is all speculation. If you want a real answer then you need to wait for an actual review of PileDriver.