Here's a look at the GL3 Unvanquished performance again, but this time using the Phoronix Test Suite to compare the performance at a variety of resolutions.
Unvanquished's Daemon engine with OpenGL 3 renderer had more frame latency spikes under Linux than with the Intel Windows driver.
Overall, the Intel Windows driver is noticeably faster than the Intel Linux driver at this point for Haswell. The Intel Windows driver being speedier is also the case still for Sandy Bridge and Ivy Bridge hardware, so don't expect any major breakthroughs soon, but the Intel OTC Linux developers continue to work broadly on improving their open-source driver's performance while enabling more OpenGL functionality and new hardware enablement (Valley View / Bay Trail, Broadwell, etc).
The only cases where the Intel Linux driver was faster for the Core i7 4770K with HD Graphics 4600 was the very undemanding OpenArena game and then for the Pixmark Volplosion GpuTest it had a narrow victory.
Aside from the Intel Windows driver delivering faster OpenGL performance, it's also still leading on delivering OpenGL 4.x support over OpenGL 3.1 as is officially compliant with right now under Linux. The Intel OpenCL Linux support is also still in its early stages with there really being nothing besides the CPU-only Intel OpenCL SDK for Linux and the questionable Beignet.
As both the Windows and Linux Intel OpenGL drivers continue to mature for this new hardware, we'll be back with more cross-platform benchmarks. For now you can expect the Windows Intel driver to lead anywhere from 10~20% as was common but with visually demanding workloads the Windows driver can be nearly twice as fast.