I honestly don't think there's any difference. Vishera seems to be a C0 revision Bulldozer. As with any new process node, you'll get refinement in design and better execution in silicon as the process matures. Piledriver = Bulldozer C0 = Bulldozer revision 2. I doubt you'll see anything that hasn't been documented in other CPU designs tbh (i.e. performance bumps between D0 ES Sandy Bridge and late D2 revision), and I'm pretty sure all new stuff they were supposed to be packing into Vishera, e.g. - the resonant clock mesh tech isn't included.