Stock spring is 18#'s for the .40, 16#'s for the 9 IIRC
Try a 16# and see how that works
That said, a magwell, and sights would be the first upgrade before springs
9x19mm, 40 and 357SIG all use the same factory spring weight of 17#.
The problem with going with a lighter spring on an XD is it may not go into full battery when the chamber is dirty and will have sluggish lockup. The reason is the fully cocked striker spring is pushing the other way trying to unlock the pistol when it is cock, which happens every time you fire the piece.
If you go to a lighter recoil spring you WILL need to go to a a lighter striker spring aswell. This will increas the chances of a light primer hit.
The gun is designed so the springs all work together. Change one and a few more might need changing.
If you want smoother get the one of the one piece guide rods out there and put a 17# spring in it. I did in my XD9 and it is much smoother now......
I called SA to order one and they told me they were all the same weight when I told them I needed a 9mm spring.not to argue, but I had a XDm .40 and got a 9mm conversion barrel for it.....I had to switch to the 9mm spring as well to get it to cycle properly
I might be off on the weights, but I know for a fact atleast at the time (2 years ago) there were two different part numbers from springfield that had 2 different spring weights
may not be the case now, but was when I had mine