AIUI, if you build a custom chip to use low-power async/clockless logic, cooling it with LN2 would absolutely make sense. It would run at high speeds while sipping power. Not for commonly available chips, though - those will waste power with every clock cycle and release that as heat. (It's gotten to the point where chips cannot be fully powered on at any time - some part of the chip will always be power-gated and kept "dark".) So any practical subambient cooling system would simply be overwhelmed.