> I've been doing hours-long CPU+GPU intensive tasks (compiling ghc, number crunching, playing games like Bioshock Infinite and StarCraft II on a HD4000) on a bunch of pre-retina MBP and rMBP 13" and they never throttled.

I find that hard to believe. I have been gaming a lot on MacBook Pro unibodies and retinas with and without an external GPUs. Especially when the internal GPU is active, the thermal load is just too high for many games and applications.

Throttling normally starts in a very subtle way by disabling clock boost and is hard to notice. Throttling below the base clock is rare and this is where it becomes really noticeable.