I've made a finding that may indicate that, at some point during development, the punches may have had those visual flashes you see in comics and action/fighting games.
When you reverse Juri's/Luis' side slam, Jimmy punches them twice in the head. I noticed before that there appeared to be a white flash from the punch, and I finally got a full view of it when I saw Gord doing it to Juri.
My theory is that this is a leftover from the beta version. I believe that these flashes appeared from every attack, but Rockstar decided to remove them to keep the game more realistic. However, they forgot the effect from the side slam reversal because it's a seldom used animation and they overlooked it.
What do you think?