I have to admit that it is much easier to achieve a silky-smooth feel to the cocking cycle with the plunger mod in place of the flat hand spring, and it DOES eliminate broken hand springs. But as done by the Italians and most tinkerers, there IS a trade-off in terms of how positive everything feels.
With the flat hand spring, I've polished the heck out of the rear of the hand slot in the frame, followed by a mirror polish on the tip of the hand spring itself and still not managed to get as slick an action as I can with the plunger mod. I'm not sure why a plunger sliding along the back of a hand is smoother than a flat handspring sliding along the back of the hand slot, but it is.
Regardless, the way the Italians and most tinkerers do it, the back of the hand has the same contour as before. And as the hand rises, the hand spring loses leverage the higher the hand goes. By half-cock, there is much less forward spring tension by the hand against the ratchets than there would be if it had a flat hand spring. And this has a negative effect on cylinder braking.
For example, on my 2nd Gen. Colt, the long angled portion on the front of the hand below the top tooth is very close to the sloped backside of each ratchet tooth as the cylinder rotates. If the cylinder tries to rotate any faster than the hand is rising, those two surfaces will engage, keeping the cylinder from outrunning the hand. And on half-cock for loading or unloading (as long as it was timed correctly), this braking effect keeps the chambers lined up perfectly with the loading port cutout. If I rotate the cylinder slightly past the half-cock "click" and let go, the cylinder will positively spring back into position, because of that nice, strong hand spring. I can rotate mine almost to the next "click", and it will still spring back into position. Try that with a modern Uberti sporting the plunger; it typically won't do that.
I know that's a "nitpick", but I like strong braking pressure from the hand, and I just don't get that with the typical plunger mod. Many here will disagree completely with me that braking pressure is important at all, but I like it. So the typical plunger mod is a trade-off between feel and function. But all is not lost.....
Other tinkerers recognize the plunger mod's weakness and do more than just remove the hand spring and drill a hole through the back of the frame; they also modify the backside of the hand, adding material to make it gradually thicken towards the bottom. Done right, this cancels out the reduction in leverage by causing the hand to compress the spring more as the hand rises. I've seen pictures of other's mods where they silver-solder this steel "shim" to the back of the hand. Looks like a lot of work to get right, but that's what I'd want done if I modded any of my SAA-style revolvers. I wish Uberti and Pietta would re-contour the backs of their hands, but if you look at them now, they look exactly the same as the ones with the flat spring--they're just missing the spring.
I am also sympathetic to Dave T's criticisms. And again, there's a trade-off. I prefer the positive feel of traditionally-weighted flat springs. Because every factory Colt I ever cycled had heavy springs, that's what feels "right" to me. I recently switched out the mainsprings and trigger/bolt springs on my Colts with Lee's Gunslinger spring kits. The action is much lighter now and correspondingly smoother to operate, but it feels "wrong". I'm keeping the spring kits in there for now, because I recognize that they DO lessen wear to the sear surfaces, bolt legs, hammer cam, and cylinder stop notches. Were I a competition single-action shooter, the question of springs would be a "no-brainer". But I'm not, and the feel of cocking a traditionally-constructed single-action is part of the enjoyment of shooting them. I also agree with Cholla that had Standard gone with a plunger handspring, there would have been howls of derision, no matter how much competitors value coil springs.
And seriously, who's gonna compete in Cowboy Action Shooting with a $2,000 revolver? I'm sure as heck not, and I seriously doubt anyone else will either. The USFA that Marshal Deadwood just sold to me had only been fired 6 times, because he couldn't bring himself to run it hard and fast in competition. And I don't blame him. If I was going to compete with a single-action, it would be a Uberti or Pietta, highly modified with coil springs and plungers, and everything else I had to do to make it fast and easy. And if I wore one out, I'd just buy another (they're certainly inexpensive enough). An older Colt, USFA, and now Standard are for lazy plinking fun and the enjoyment of shooting something traditionally and finely crafted. They can certainly be pressed into service if need be, but their primary purpose is for the former.
But since this is a Cowboy Action Shooting forum, I'll have to defer to the preferences of those who actually compete in the sport this site is all about. Frankly, it is a nice surprise that there are even subforums here for Colt and USFA, as most competitors won't actually risk accelerating wear on expensive single actions through competitively shooting them. Dave T was quite the intrepid competitor for using original 1st Gen Colts in CAS matches, and I know that others have and do this, but I bet they don't care all that much about speed when they're doing so.