End-to-end latency is definitely something that deserves attention. It'd be interesting to modify mouse&kb by adding LEDs in series with the switches. That way you'd get good visual indicator when the event happened without any added latency. Combine that with 144hz monitor and actual high-speed camera (I hear good things about Chronos camera), and you could do very accurate software latency measurements
This MSR video from few years back is also pretty cool wrt touch interface latency: https://www.youtube.com/watch?v=vOvQCPLkPt4