Anxious to hear... Not really surprised they did nothing to the gun. It's got to be a magazine issue, but I don't believe springs are the complete solution. We'll see.
Well, here is my report. I'll try and lay out what I did.
S&W sent me back the mag I had sent with new springs, and springs for the other three. All of the springs had yellow paint near the ends. They were roughly two links longer than the old springs. When I replaced them, it was obvious that they were much stiffer. One, in fact, was so stiff that it was almost impossible to load the last round. I loaded them up on a Thursday and put them in the safe. Went to the range on Friday.
First ammo type was Federal American Eagle 90 grain. Loaded all mags up. Stovepipe on last round 3 out of 4 magazines, stovepipe on next to last round on one. Not a good start. Loaded them all up again. Stovepiped on last round on only one magazine, rest were fine. Isolated this mag.
Loaded 4 mags again, all fired fine except noted isolated mag which stovepiped last round.
Did it again, same result with the isolated mag.
Down to 13 bullets, loaded one with 8 and one with 5. All fired. Did not use the isolated mag.
Switched to Magtech, had a box of 50. Loaded all four mags.
Isolated mag stovepiped next to last round. Of the others, one stovepiped last round with only 5 loaded in magazine. One mag failed to lock slide back after last round fired. The above mentioned isolated mag also failed to lock slide back after last round fired.
Switched to Fiocchi 90 grain.
One stovepipe in first three mags (not counting bad mag). The bad isolated mag failed to lock slide back after last round. On a subsequent loading it "stovepipe-ejected" the last live round. All the rest were fine.
So of the 200 rounds I fired, what did I learn?
I have a very consistent live stovepipe on last round with one mag. Maybe it's the follower or the feed lips- not sure. I did note with this magazine it was extremely hard to load the last bullet. In any case, I am taking this mag out of the rotation.
Of all the others, nearly all the live round stovepipes seemed to be the very first time I used the magazines with the new springs.
The failures to lock back are a little odd. Most occurred with the bad mag. One occurred with one of the others. I would think this might be a function of the recoil spring.
Next time I will do more testing. I feel the gun is stovepiping less when I factor out the bad magazine.
One last comment. I was explaining the issue to the range safety officer. He knew of the problem with the EZ and said that when loading the magazine, make sure to slingshot the slide- don't just release the slide lock lever. He claims this cures the problem. I tend to use a mix of slingshotting and slide release. I wasn't paying attention to this but will be careful to only slingshot next time.