I can look into ammo counts of certain levy units relative to the damage they do to Cohors and the like. Trust me when I say that usually when a ranged unit is overperforming, it is usually because I gave them too many shots.

There was a long time ago where this was the case with javelin units.

As for getting a decent unit of cavalry before I finish my testing, the easiest way is to vassalize a desert tribe. I believe they get one from second level settlements.