I think if you're pressing multiple times per second, it won't make that much of a difference.

Personally, I used a stopwatch and spammed for 15 seconds.
Counted how many times I had pressed the key.
Personally I click twice per second, on average.

If you click say 2 times per second, then at most you wait half a second for something to happen because of the GCD.
If you were to click 5 times per second, a toon stuck on the GCD would only be delayed at most 0.20 seconds.

No system is perfect, but click macro's are far and away the best solution so far.