That's a good spring to run in a 7.000" extension, and tame a .308AR, so it would be a good choice in this application. We all know that .40 is just "10mm lite," and it recoils like it.
At SHOT this year, I hit the Sprinco booth up - the whole crew did, 4 of us. We spent over an hour there BSing with those guys, and talking shop, talking specs - they get it. Look into their "convertible buffer." It was close to release during the show, and it's badass. It's atake-apart buffer, with all the weights you'll need, so you can tailor that thing any way you want to. It's pricy - but it's every buffer-weight combo you could think it. I still can't find it on their website, so you might have to call them about it.
That 9mm buffer you got - it's longer than regular 3.250" carbine buffers, and heavier, right? I have two pistol buffers that are 8oz, and those are doing a good job - but on 9mm and .45ACP carbines. That ain't .40.