10% better in multithreading than SB 4c/8t sounds too low.
you state that single thread perf will be much higher than thuban, but keep in mind that if its 10% faster clocks, 10% higher IPC (after the loss due to 2 cores in one module), and 33% more cores than thuban, it should be 60% faster than thuban in multithreading (think cb11.5)

if you believe that 1 core vs 1 core of BD vs SB will be pretty close to each other, then why do you think 8 cores will struggle against 8 threads?