It can't handle the 750m, it actually drains power from the battery if you try to use the 750 at max.
You're right. And the throttling because of that really hurts performance.. Some people say a 650m can get up to gtx 660m levels.. that's funny
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Never said it is. Still, peak power consumption is limited by the TDP (by design).
TDP should not be used as a method to rate a GPU/CPU's power consumption.
Here is a 3840QM with a 45watt TDP drawing 66.9 watts..

TDP is just used as a spec for designing heat sinks that can properly dissipate heat without throttling. Apple usually misses this memo (or maybe they did on purpose by bringing down the TDP by using a lower wattage power brick?)