Never heard the expression "teaching your grandmother to suck eggs"? Actually that's probably for the best as it doesn't make a lot of sense. It's supposed to mean telling someone something they already know but what's that got to do with sucking eggs? Strange.
Anyway, yes, the velocity under the car will ganerally be greater for a car with diffuser than for one without. For real cars there can be several other factors which strongly affect this as well but the principle is still correct.
What causes there to be a peak in the low pressure at the apex of the floor/diffuser intersection is the way the air turns to follow the surface of the car, which is now angled upwards. I should clarify that the peak suction occurs on the car side of the 'pipe' and *not* across the whole width, though it may extend quite a way across. What happens isn't straighforward to explain - but i'll try so please bear with me but tell me if it's not clear or doesn't make sense.
The air is compelled to stick to the car body by virtue of its (low but very important) viscosity. Also it is still subject to Newton's laws so to change it's direction there must be a force acting on it to change direction. If the corner is not too sharp, the viscous effect between car-air is enough to pull around the air immediately adjacent to the surface but the air-air viscosity isn't enough away from the surface and so something else has to do it.
A pressure gradient is required within the air perpendicular to the diffuser apex to push/pull it around the corner. Pressure and speed are intrinsically linked via an often mis-qouted relationship derived by a chap called Bernoulli. Bernoulli's (complete) formula shows that a fluid can 'borrow' energy from one form of pressure and lend it to another. In this way speed and pressure are inversely proportional to each other (with a couple of caveats which i won't go into here).
The required pressure gradient thus forms via a speed gradient within the air. The air near to the surface but not immediately adjacent has to turn through the greatest angle so it needs the greatest pressure force acting on it, hence the speed there must be the greatest. Further out the turn radius is larger so the change in pressure doesn't have to be so big and so the speed change is smaller.
You get the same effect wherever air flows around a convex surface but where the curvature is not so severe that viscosty isn't enough to provide the force necessary to turn the air immediately adjacent to the surface around that tight a radius. Hence you get faster airflow and lower pressures around smoothly curving surfaces where the air has to change direction.
This is the same as what happens around wings which are essentially posh turning vanes. It happens at the front of the bonnet of your car, around the sides of the front bumper, over the top of the windscreen, around the A-pillars (though often rain gutters mess this up) - bascially wherever the air can still make the turn around the corner. Where it can't, it will head off in something vaguely resembling a straight line.
With a race car diffuser they often make the flloor/diffuser junction as sharp as they can get away with so that the air has to make the tightest turn and you get the lowest pressure at that point. This is great for peak downforce but can make the car very sensitive to small changes in pitch, heave (ride height) yaw or the presence of another car up ahead. Often you have to take a less aggressive approach and use a larger radius (still tiny in real terms) so that the air will still be able to make the turn under a wider range of upstream conditions.
Is that any clearer of have i made things worse?
