The idea of polishing a stock AR trigger falls into the same category as several other things AR builders like to disagree on like- proper way to hold the upper receiver/barrel when torquing barrel nut or muzzle device, whether you check head spacing or not, do I need to stake the castle nut on the receiver extension, etc... etc....
It depends on who you ask.
If the polishing is done correctly and by someone who is competent and careful, I don't see a problem with it. I have several rifles with self-polished stock triggers in them and I've never had a problem. I have documented how I do mine:
http://acatholicmechanic.blogspot.com/2016/07/improve-your-triggerfor-next-to-nothing.htmlMy opinion on JP reduced springs- NO. They will take your trigger pull down to 4 to 4.5 lbs but at a cost- light primer strikes. I have taken these out of all the rifles that had them and put stock springs back in. Too many issues with fails to fire because of light primer strikes- even with various domestic ammo.
In my experience- any hammer/trigger spring combo that yields under a 5 to 6 lb. trigger pull, you may or will have light primer strike issues. (when using a mil-spec FCG, polished or not)
I've seen stock AR trigger/hammer springs in the range of 6.5 lbs. (Palmetto State Armory) to 9 lbs. (DPMS), so it really depends on where you buy from. Wolff Gunsprings makes aftermarket springs.
This is just my experience. I be glad to hear the experiences of other who have done the same experimentation.
Edit: typos