I've done tons of dry fire practice. I've practiced regularly. I've improved my groups a bit, but I still pull about 20% of my shots to the left. I'm putting tons of finger on the trigger. I've read some folks who suggest that flat triggers, such as the overwatch triggers, are helpful for people like me who struggle to not shoot glocks to the left. I'm curious if that has been anyone's experience here? Should I consider getting a flat trigger like the overwatch TAC?

In short - no, they don't.  

I used to shoot right (i'm left handed) with my stock G19.  Frustrating, but it is what it is.  2015 was a good year, so that glock went off to agency for a makeover.  It got the full workover, and while I wasn't expecting to all of a sudden shoot better, I hoped that the improved ergos would possibly help.  The flat trigger feels great, but the idiot behind the trigger was still shooting off.  

Fast forward to 2016, I picked up a Gen 4 G17 on a whim.  For some reason, I started to find the center without adjusting or compensating for the drift.  Probably has to do with the fact the Gen4 has a slightly shorter distance from back of grip to the trigger and my medium sized hands had less issue with a proper trigger pull.  Ended up with a Zev trigger setup, but also toyed with the Agency trigger.  Made no different which one I used.  Boiled down to the preference for a curved trigger in the end.  

At a course later on in 2016 with Ronin Tactics, Tu stressed having a strong support hand grip to drive the gun.  I've always tried having even pressure with both hands, but after I started putting more effort on a strong support hand grip and looser (but still firm) shooting hand grip.  By doing this I've felt that it's allowed the trigger finger to shoot with less input, just a straight back & forth motion.  Might not be the answer you're looking for, but I hope it saves you a few dollars for buying somethign which you hope will change your results vs something that will change your results.

I've noticed the same thing, that reduced strong hand grip pressure combined with a "death grip" with my weak hand tends to fix it. But I'm too wimpy to sustain that.

Ill chime in with another, NO they do not. They may to some slight degree, but not enough to "fix the problem." I have several flat triggers, and while I much prefer them to factory curved Glock triggers, I will still pull shots left if I don't fully concentrate on my trigger press.

With my g19 with factory trigger I also tend to shoot left unless I'm really, really concentrating and I would still pull a few to the left anyways.  I've been running the Zev trigger in a g19 and g17 and they have helped me tremendously (pretty much completely) on stopping shooting my glocks to the left.