In competition guns, people will change a captive recoil spring to an open ended one or user-replaceable captive guide rod spring of a particular poundage. This allows the competitor to load their own ammo with light powder charges to just meet the power requirements but still have their pistol function well. The light recoil spring lets the slide fully retract with a lighter charge, still get ejection of casings. Then when they find their desired load / recoil spring combination they fine tune it for accuracy and stick to it. This is usually done in full size pistols.
A light recoil spring under full power (normal) or defensive +P ammo would batter the frame parts eventually, and increase felt recoil as the slide wasn't slowed before it came to a full stop at the end of travel. There may be failure to feed and trouble returning to battery in adverse conditions where forward motion of the slide is hampered.
A *heavy* recoil spring guide rod like those made of tungsten increases weight near the muzzle. I haven't seen any reviews where they could clearly tell a difference before and after.
If you are considering wanting the Shield 40 to have less muzzle flip, my suggestion would be checking your stance, grip, and a grip modification. The Shields are thin, but the down side is there is less for the hands to clamp down on, and the texture isn't as helpful as it could be. On my EDC (kahr cm9) I put a bike inner tube, then a Hogue Handall Junior on top of it, which gave it a palm swell, finger groove, widened the grip, and has nice texture. The inner tube keeps the grip from slipping down. I have also used rubber Talon Grips on other pistols and they help.
Stance: I used to shoot both feet even to each other. Now I put 1 foot half a step in front of the other and it made a world of difference.
I have a M&P40c I got a conversion barrel for, but in the end wound up purchasing a 9mm slide. I got it in 40 cal out of curiosity, not really because I was a fan of the cartridge. After trying it in IDPA I had a desire to shoot faster. I have shot a bit more since then so if I were to try 40 cal again I would probably be able to shoot faster with more accuracy than before. But it took deliberate practice.
Did I *need* to do grip enhancements? No, I could shoot accurately enough with them, and fast enough (I believe) for defensive use. But when I wanted to work on shooting faster just because, it helped.