Not my PR, but I saw a Hawaiian rugby player (near Pearl City) do a little over 315 lbs for step-ups. Around 4 to 5 reps if I remember right, and at a fast pace.
Said he switched over from heavy squats because they bothered his lower back, and never really felt right for him. Never any problems with step-ups for him (me either). That was a couple of years ago, probably much more now...no doubt. He must have weighed around 205-210, or so. Might suggest DB's , rather than a BB, for most folks.