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!