No experience with the .40 but I did try a couple in my 9x19 M43. The fit wasn't too spiffy, leaving an approx. 1/4" gap between the grip frame and the finger rest. This, coupled with a rather loose fit in the well allowed a good deal of movement which made reliable feeding almost impossible. During recoil the feed path would drop with the mag's movement and run the nose of the next round into the ramp nearly every time.
I went back to the stock 7-rounders and curling my little finger under the base. Perfect functioning trumps an extra cartridge every time IMO.