The Mirage 5 is a versatile aircraft, but has a high approach speed of 180 knots and a high landing speed, requiring long runways.

Underwing flaps, like those of the Gloster Javelin, would surely work, and the fact that they are close to the CG would not cause an acceptable pitching moment.

Tested on a small free flight glider, the flaps on a delta resulted in a noticeable increase in lift and decrease in turning radius, the elevons were not overwhelmed by the downward moment of the flaps.

I would think the system would work, however my question is what sort of performance effects: increase in weight, for example, and possibly drag, would result.

Later models of Mirage incorporated canards and other devices to reduce landing speeds: my question is, maybe Dassault Aviation overlooked a design feature or did they evaluate it and decide that it would not be worth it?

I would have incorporated these in any delta wing fighter I designed, at least to offer it as an option to customers like the tailor made canards were to be offered.