OK I see where you are coming from now. The issue is that if you run the exact same video through any console twice (for example, a stored replay of Ridge Racer 7) you will always get a very small variation in the results. This could be down to any number of factors - for example, background processes related to the OS. So it would be very difficult to measure and say for sure it would be down to SPU latencies.
Other question for the same PS3 is always the same SPE(s) dedicate to OS, or these change after reboot?