There are factory 92 mags that are 17 rounders. They put them in with the 90-Two and the 92A1. They WILL work with the 92FS.
I have 7 of these mags. I did have to have Beretta exchange 1 of these earlier this year because of a bad magazine.
I personally use the factory 15 rounders for self defense. They are tried and true. They shorten the follower to squeeze in the 2 extra rounds (the baseplate is also a tiny bit longer as well).
I started a discussion on this issue in Feb 2011 here:
Seems that many agree with me...