First of all downstop means the limit on the downtravel of the suspension (droop limit). I saw somebody using up and downstop the other way around (that is maybe more logical but not the "widely used" explanation).

Rear downstop has most effect on braking. In general more downstop (less rear downward movement of the suspension) gives more stability under braking. But limiting suspension movement is generally less good in bumpy conditions.

Front downstop effects the on power steering. If you run more front movement you get a little more on power steering (the front wheels stay on the ground longer). But if you go too far the car's response slows down and could get vague out of the corners.

In a small track with lots of corners you'd expect more downstop (less suspension travel) gives best results because the car responds quicker. Try to always keep the car "in the springs" so if the car is stationary on the table you can lift the chassis a little before the wheels come off the ground.