Bollocks. That is just not true. Something in Windows is pegging the chip to its p5 state under iGPU load, period (probably a combination of ACPI drivers and microcode in the CPU itself, but still, I haven't been able to get iGPU-induced throttling to the p5 state to work under Linux. The Linux ACPI driver won't allow it/won't do it, even without me using cpufrequtils).
All the noise about Kaveri being TDP or cTDP constrained stem from people trying to push past 4.5 ghz on Gigabyte and/or ASRock boards, the p5 state throttling, and some people drawing erroneous conclusions.
Here is the bottom line: Under Windows, set your p5 state to be where you want it to be and the CPU will not throttle under heavy iGPU load, even if the CPU is also under load. I can push my 7700k to 4.7 ghz and 1028 mhz iGPU, load up y-cruncher or Prime95 or whatever and run Furmark in the background and not see any loss in performance. It'll run hotter, sure, and it might even crash under all that strain, but the chip isn't going to just start to slow down arbitrarily on account of a TDP constraint.
If you want to go past 4.5 ghz on Kaveri, you pretty much need an a88x-Pro or Crossblade Ranger (or maybe the A88x-Plus, but I make no guarantees.).