For all we know they've been testing it for a while... MS learned their lesson very hard this gen with the RROD. I don't think it will happen again.
Overclocking the CPU at the last minute without testing stability before producing tens of thousands of units sounds like a good idea.
After all, that's why the X1 is quite large. It has a really beefy cooling system that's also whisper quiet. That's why they were able to increase the GPU clock speed, and they apparently also felt confident in increasing the CPU clock speed now too.
Great news for my preorder. Nothing bad about getting a slightly better system!