In level flight at 400 knots, what happens when the speed brakes are extended?
Because the speed brakes are tail mounted, and because they deploy upwards and downwards equally, pitch should not change. The airplane's fly-by-wire system likely dampens any slight pitch changes caused by the buffeting.
The only photos I've seen of the speed brakes open are of them fully open, not partially open. So they may open gradually with different deflections up and down, or with different resulting air loads up and down, or both, or neither. But in any of those four cases, the FBW should correct for pitch excursions by deflecting the other control surfaces.