Which Zev kit did you buy? The ult kit is great. Make sure you change striker spring, plunger spring and connector. You will need to adjust take up and over travel. if it is still really heavy than you have another issue going on.
Mcnally Trigger bar, OEM minus connector and 25 cent polishing. Dont mess around with trying to get too crazy with plunger spring and striker spring. This set up is pretty good.
I run a light zev on my gen 4 17. only issue there is if you run a back strop its a tight fit and will warp the bs, even with fitting. Not an issue on a gen 3 gun. Gen 4 I would go this a TTI
It all depends on the shooter and what you are doing. I am set up as a direct co on my patrol rifle but keep my rear down. Front stays up so you can snap shoot using aimpoint as a large ap.