I have a std 20" barrel Mark II GL ( I'm a lefty ) and I got a FV-SR as a loaner gun for my guests here at the ranch.

Both are tack drivers, and there is little benefit to the longer barrel.

If your intent is to add a can then you might be using sub-sonic loads - which again the longer barrel may even cause some loads to go over the sound barrier.

At some future date I may look into getting a FV-SR barrel fitted to my Mark II.

I think I paid $195 for my FV-SR from Bud's Guns last New Years Eve.

BTW buy extra 10rnd mags as the FV-SR comes with a 5rnd that I hear is somewhat finicky. I will say the few times I've used the 5rnd its worked fine for me - I just usually use the 10's.