Hard to say mention as SR FX...

1) Vishera FX is good CPU for almost 2 years old CPU...OC ability up to 5 GHz
2)in theory FX Steamroller 28nm could be similar in TDP, but with lower clocks potential - with some luck up to 4.5 GHz. With some improvements is Steamroller B very awesome against Piledriver, but in other cases are only few procent or the same as Piledriver or a bit lower (of course, clock to clock is SR better tahn Piledriver)
So 4.5 GHz FX SR could be average at similar performance as 5 GHz Vishera FX. In wprime 1024 40% better, but in winrar 20% worse etc...

28nm SMP is not good now for big high clocks cores. Maybe Excavator core in future or soemthing new.