L4 cache only helps in memory-intensive workloads, such as gaming, compressing and decompressing, certain types of simulation... But it does little to nothing to those workload more calculation-intensive.
Most single-threaded benchmarks out there are from the second group. And those memory-intensive workloads tend to take advantage of multi-threading.
For dx11 gaming the 5775c should still be better than Broadwell-E, and in dx12 the extra cores put Broadwell-E the first place, although the 5675c and 5775c shouldn't be that far. It has been proven memory plays a crucial role, in such a way a 5775c can take as many draw calls as a 5820k.
I'm so impatient for the new 3dmark dx12 benchmark, it will be funny to destroy those 6700k and 4790k heavily overclocked xd