[AeroMcAeroFace]

No it isn't critical, the diffuser only needs to go to the bumper edge, but the further out it goes the better the downforce. There is also some potential benefit in aerodynamics extending it further but that really depends on far too many factors for me to give recommendations on how far because I don't know what the research says.

Things like spoiler positioning (if any), roofline shape, diffuser angle, expansion ratio all can affect how far out may be optimum for drag or downforce.

But if you are worried about ground clearance then just go to the bumper, no side fences needed.