He has a basic job that he recommends for SHTF rifles where he modifies the friction points and the stock springs. Then there is another job where he will switch to liter springs to get an even liter pull, and modifies the hammer to try and speed it up to get the momentum needed for harder primers. Heavier springs should help, certainly worth a try.
I got the basic job from him and its really good. Not super lite, but no take up at all, just a clean crisp break. I'm not sure which I want to get for my other rifle, but I'm thinking about getting another basic mod job after reading this thread. Springfield says the speed hammer ensures positive ignition on harder primers when using lighter springs, but I've seen others say a speed trigger can cause lite strikes without even mentioning liter springs.
ETA :
http://www.triggerwork.net/ar15s.html
Option 1 :
4lbs pull, very short, smooth, clean break. $35
Option 2 :
For 3lbs pull price is $50 (new match spring set installed; hammer tail removed ("speed hammer") included in the price for more positive ignition for hard military type primers.
I recommend the 4lbs pull for duty and field rifles, 3lbs works perfectly for long range and target style shooting.