I'm not sure where you're pulling those performance numbers from. Because of decreasing scaling, given the same IPC, roughly: 8C/8T=6C/12T.
Where in the world did you come up with 8C/8T = 6C/12T? Based on what data? Scaling doesn't change with more threads if the program actually uses 8 threads. An 8 core i5 760 would be 90-100% faster in a program that uses 8 threads instead of 4 vs. a i5 760. If you argue that few programs use 8 threads, then I agree, which is why BD being slower in 1-4 threads is critical.
Is the Core i7-990X 2x faster than the Core i5-2500K?
i7-990X is a not a SB architecture and it only has 6 cores. So how would it be 2x faster than a 4 core SB? Your comparison makes no sense.
I would say that it is possible that an 8-Core 3.6GHz BD will be 50% faster than Core i5 2500K in multithreaded apps.
You are contradicting yourself. An 8 core Nehalem would be way faster than just 50% faster than a 4 core 2500k.
Under your scenario (~50% faster), it would have
worse IPC than Nehalem/Lynnfield (since if it was = 3.6ghz 8-core Nehalem, it would be almost ~100% faster). Remember 8 divided by 4 is not 50% greater, but 100% greater mathematically:
3.6ghz Nehalem = 3.13ghz SB (since SB is 15% faster per clock).
Then 3.6ghz 8 core Nehalem would be be nearly nearly 90-95% faster than a 4 core 2500k @ 3.3ghz in an 8 threaded app. Even if we discount perfect scaling, it would still be 70-80% faster.
I never said it's not possible to have 50% faster performance in multi-threaded apps since it's obvious AMD is focusing on multi-threaded performance. What I did say, however, was that there is no way FX-8100 = 8 core i5 760 @ 3.3ghz based on simple math.
Well, Cray is upgrading 12 core MagnyCours chips with 16 core Interlagos. I'm not in charge of large deployment but I'd imagine they'd want at least a ~25-30% performance bump for the expense and effort of an upgrade. Which would mean the output of a BD module would at minimum need to be equal to 2 previous cores. If they couldn't offer that in the same TDP not sure they would have been able to sell them. A bit worrying it's taken several delays to get there though.
16 / 12 = 33% more.
So in that case 1 BD module = Phenom dual core. That implies pretty much identical IPC to Phenom II.