You did not mention your level of proficiency or shooting experience. A just adding a 6" (longer) barrel will add velocity but will not increase the accuracy of the pistol. Also a longer barrel does not increase the sight radius and so will not assist the shooter in aiming more accurately.
A drop-in fit barrel can only make minor changes in grouping. If you can already out perform the accuracy capability of the stock barrel, then a custom fitted match barrel can tighten your groups, especially if you also add some target style adjustable sights and an improved trigger. The XD was designed as a combat pistol and will not have the accuracy of a target pistol even with a match barrel. The loose fit-up of the slide and frame (and other parts) are intended for reliability and will not be changed by adding a barrel.
If you do want an extended length custom fit match barrel at reasonable prices, check pistolgear.com for a barrel to have installed or Springer Precision for custom work (or other site sponsors). StormLake Barrel, EFK Firedragon and Bar-sto sell both drop-in and match type barrels that can be custom fitted.
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