There definitely isnt a hard cap that i know of, ive been oneshot with like 60+ hp heavy armor. From experience it seems like its nearly impossible for infantry to reach the same damage potential using the same speed bonuses against cav, or the window is extremely precise.
That said i have had couches deal suprisingly little damage at times (like 70% with 7 IF 66b armor), but i think that comes down to either builds or me negating as much speed bonus as possible.
Is this an honest buff request? Your cav KD must be like 3 or 4 right? All you really do is just find unawares and oneshot them, or continuously play on the safest angle and hope that they dont have the athletics to maneuver it for a counter.