Max f/4 Lens + TTL Flash
Pre-flash measurement is taken while the lens is wide open, just as you view the scene thru the VF. In fact, you can see the pre-flash in the VF. So, a f2.8 or faster lens should yield better measurement.
The actual flash occurs after the mirror flips up, the lens stops down and the shutter fully opens. Thus, shooting at f5.6 has nothing to do with pre-flash with a f2.8 lens.