Do normal cows at level 19, then go level someplace else (eg. Arcane) and do cows again at 20.
The exp gained from a single normal cow run is dramatically different at level 20 when compared to level 19. Starting at level 20 it's not unusual to finish the cow run and have gained two levels. At level 19, you'll be lucky to move half an exp bar.
The cows are HIGHER level than you in both situations. So yes, there is a penalty for killing stuff above you. A huge one.
You all are aware that you shouldn't be doing Baal runs in Normal before 25, right? Why do you think that is? That's right.. it's cause the exp sucks!!! But.. but.. they're higher level than me!!
It makes sense, I mean, imagine if there wasn't - i'd take a level 1 straight to Hell Baal.
The reason why level 1's in Hell Cows used to work in 1.09 is that the cows gave SO much exp and that lowbies required so little to level that even with penalties, they levelled very fast.