cc @dave who may know
Sorry for the delay! I meant to respond sooner but I hesitated because wanted to make sure I said the right thing, and then it flew out of my mind completely. 
Okay, the most important thing: We haven't yet shipped any units with the new CPU. We're still using up the last of our inventory of STM32F746 on the original mainboard. Those will run out in the not-so-distant future (no, I don't know exactly when) so we're busy updating the firmware to support the new hardware and validating the design so we can keep production moving.
At this point, I personally don't think it's very useful to give a bunch of details on the hardware change--I know you're all curious but it'll all be public soon enough, and talking about hardware always seems to invite even more questions that I don't have good answers for. 
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The CPU is capable of 216 MHz if you give it enough power, but to save battery we're using a lower voltage which requires us to slow it down a fair bit--I think we're somewhere around 160 MHz if I recall correctly. And yes, the data sheet says it's got 4K of I-Cache and 4K of D-Cache.
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thanks, we appreciate the insight, sorry for pestering! 
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