I knew about the P10 and P07/P09 magazines. I didn't really mention the P10 as I've read/seen pictures of the slight difference in the way the bases look when swapped between pistols. I don't know if that's a difference in the magazine shell/body or just the base itself.
Anyway, the picture I took of the extended base on the P09 magazine. For 100% clarity, this is a P09 9MM magazine in a P09 .40 pistol. The mags. fit the same in the two caliber pistols, just wanted to state things up front. I have not shot the P09 with the base/spring kit. You do have to remove your magazine follower from the original spring and put it on the new extended spring.

If these work out okay I'll be ordering some more. Oh, the reason I ordered from Midway was one of the members on the M&P Forum said he ordered a couple from the company that makes them and when he had one of the springs fail (twisted up badly when loaded to 23 rounds of 9MM) the maker company refused to do anything but offer to sell him a new spring stating it was his fault (poor installation.) I believe if I have an issue Midway would be more likely to supply a new spring, or a new kit. Or I'd just trash the factory kit spring and order an extended spring from somewhere else. I need to check to see if Wolff Springs sells extended magazine springs. I know I've seen them for sale somewhere, just didn't need any at that time.
[Mods enlarged image]