I use them with my P357. No new springs in mine. The only thing I do to ensure good reliability is to only put 9 rounds in the carry mag (+ one round in the chamber) and 10 in the spare mag -- even though they will both hold 10-rounds. The thought being -- after the last shot of the carry mag, the slide will be locked in the rear position. Placing the loaded spare mag in and releasing the slide will strip the 10th round from the new mag, leaving 9 in the mag.
Why do I do this? The +1 extensions will give you the ability to hold 10 rounds of .40/.357Sig -- but it is a tight fit. A full 10-round mag will lock in the KT, but it too is a very tight fit requiring significant upward pressure to get it to lock in. Myself, and some others have experienced feed problems with a full 10-round magazine. Once the capacity is down to 9-rounds -- things work great.
So, I sacrifice 1-round of "capability" and effectively carry 20 instead of 21 (two full mags of 10 plus one in the chamber). But, I know that I will not have a FTF after the first shot!
As I've posted before - I'm not a big fan of the P-40 mags or +1 extensions. I use them loaded with 10 rounds just like flyandscuba - only for backups. I carry my P-40 with a P-11 mag (8+1) that has a finger rest. It works for me. Others have not had the same success with P-11 mags that I have, but I do have Wolff extra power springs in the mags so that might make a difference.
I had more reliable feeding using P-40 mags that did not have the +1 extensions. No problem with 9 rounds. That 10th round with the +1 extension is just too tight to strip off reliably. But get this - some ammo works better than others and I never had a problem with Win value loaded 10+1.
See my thread on why I prefer P-11 mags because of the more stable follower. I did a little P-11 mag feed lips spreading way back when I first got them, so that may be why I have no problems with them feeding .40 S&W in the P-40. It certainly made them feed 9mm in the P-11 a lot more reliably.