Hi OMGWTF,
You asked:
Is this worth working on? Or is it just easier and superior to lead with a jab, cross or x2 jab?
I'm a southpaw like yourself and years ago I spent some time training on a lead overhand right out of curiosity as to its effectiveness. The result? I was able to occasionally utilize it as a counter over an orthodox fighter's jab but you know what? This was against inferior competition. When I faced equal or superior competition, it was a shot that never came out of me. Anyway, I scraped this blow from my arsenal long ago because this punch just didn't "flow" and feel right for me. But hey, that just with me. You may want to spend some time with it and see for yourself if it "fits" you. There's one caveat to that though, it does help to have long arms. If you're a short armed fighter, I wouldn't waste precious training time on this shot. You'd be better off spending the time perfecting a counter double jab, counter straight left hand, or a counter lead hand (i.e. right hand) shovel hook to the spleen (which you should also perfect for combo attacks as well).

On a side note, MMA legend Fedor Emelianenko loved this shot and is the only fighter that I can think of right now (in either boxing or MMA) who used it with any frequency. Check out his fights on YouTube if you're interested in seeing how he used it.

Take Care,
Lito