My brother and I both have the 9c, his son has the FS.40, and wishes he would have got the .40c.
I went and bought a FS mag, w/sleeve and its great for around the house, or as an extra mag on the range.
I would stick with what you have, and just get the extended mags, w/extension sleeve(that fits over the mag) and voila, you have a FS handgun GRIP, with a 3.5in. barrel.
Believe it or not, the longer barrels, are really not all that more accurate than the 3.5 in.
Unless you are really into competition shooting, this would serve you fine.
Personally I would go get the .357 Sig Barrel and you would just swap it out with your .40 and you'd be good to go.