I installed a Glock 3.5 (-) connector in my G19 (before I sold it!) and found it offered a minimal improvement.
A friend of mine installed the same connector in his G17, but he also did the .25 cent trigger job mentioned above, which is basically polishing contact points.
Again, there was another slight improvement over just installing the 3.5 connector. However, doing both mods did improve the factory trigger.
If you want to really improve the Glock trigger pull, break and reset, IMO a complete trigger drop in kit is the best way to go.
I have a Vanek Classic drop in trigger kit in my G17 and it offers a safe, light pull, crisp breaking trigger pull with a shorter reset.
The Zev Tech drop in trigger offers more adjustability and from what I have read is also a great trigger kit, but it costs a bit more than the Vanek.
Jon (OP), I don't know where you are located, but if you are in the Toronto/Hamilton area you are welcome to try the Vanek out. PM me if you are close to me and interested.
Cheers