I think the difference between 50% and 5% might be the difference between marketing and engineering. Engineers tend to be very literal.
If 2 cores get you 180% performance of 1, then in simple terms, that extra core is 50% that gets you the 80%.
What I asked the engineering team was "what is the actual die space of the dedicated integer portion of the module"? So, for instance, if I took an 8 core processor (with 4 modules) and removed one integer core from each module, how much die space would that save. The answer was ~5%.
Simply put, in each module, there are plenty of shared components. And there is a large cache in the processor. And a northbridge/memory controller. The dies themselves are small in relative terms.