6 module part would have drastically improved core (SR), not just 6 PD modules.
AMD states 30% higher ops/cycle throughput so 50% more cores(or threads) with 30% higher throughput (20% higher "IPC" x 10% higher MT efficiency due to split decoding stage) would equal 1.95x or practically 2x the performance of 8350 if the new 6M SR based FX would run at ~4Ghz @ 28nm.
So there you go, if ^^ above would to happen it would cream it. No doubt about it.
Maybe I'm reading this wrong, but this seems contingent on the idea that 6M Steamroller will use as much power as 4M Piledriver at the same clock speed.
If that's what you expect I think you're overestimating the performance improvements of GF's 28nm process by a lot.
I expect the transition to be somewhat like going from 32nm to 28nm on Samsung's process (given that they're both common platform). Which isn't that big of a deal - Anand guesstimated 15% lower power consumption at the same clock speed, which seems about right with what you can see in Exynos 5250 vs 5410 and Samsung's promotional numbers. In AMD's case you have to factor in moving from PD-SOI to bulk, which I expect to hurt power consumption more than it helps. And I expect an SD module implemented on the same process to use more power/MHz than a PD module - I don't believe AMD got all of these performance improvements from free. So I just don't see a 50% power reduction at the same clocks happening. I'd more readily the power/module to be about the same for the same clocks.
Now if this 4GHz part is supposed to be one of these 220W monsters then it's a different story.. but who knows what thermal envelope Intel will release their 8C Haswell-Es in. Nevermind the 12C parts.
And if SR really did deliver 2x the peak throughput vs 4M Vishera how would that make it cream 8C Haswell-E if that also clocks about the same as the 4C one? 4M Vishera doesn't cream 4C Haswell, and on average 6M SR will only scale worse because it needs 12 fully loaded threads to achieve that throughput.
Personally, I don't think AMD will even do a 6M Steamroller part, at least not one that isn't an MCM. The demand in the desktop space is going to be too low, and I really can't see AMD doing a server-only die now - they never have in the past and their server department looks like it's shifting focus from their high end parts.