I don't have an EFK or an XD, but I do have an XDm Comp with a hand-fitted Bar-sto match barrel.
I have shot both barrels one after another, in a "single blind" (meaning a friend switched the barrels and didn't tell me which I was shooting) test, 100 rounds with each barrel. The match barrel, hand-fitted by Daniel at Powder River, gets me about a 5% reduction in mean radius of group sizes.
That ain't a lot. For clarity, with the standard barrel (which, on the XDm, is supposed to be a match barrel already, so YMMV with an XD) I'm getting groups with a radius of about 2.65 inches at 25 yards. That's freehand, but slow-fire.
A 5% reduction is about an eighth of an inch. Sure enough -- I'm getting about 2.51 inch groups with the custom barrel.
FWIW, that's exactly what Daniel predicted. When I was ordering, he told me an XDm competition will usually see improvements around 5% if it's in good shape.
So if an eighth of an inch is worth hundreds of dollars to you, then go ahead. I, too, like mods, and I also compete, so that 1/8th can mean the difference between an A and a C score to me. There's also the issue of the custom fit barrel seating itself better during rapid-fire, but I haven't measured that, and probably never will.
What I don't know is how much of an improvement a custom barrel will make in an XD. If we believe Springfield when they say that an XDm Competition already has a "match" barrel, then it's expected that the best of the best would only make a minor improvement.
Since an XD does not have a match barrel from the factory, then perhaps we might see a larger improvement in results, making the mod a better value. How much more, I have no idea at all.
One last thing -- Daniel told me that much of the improvement does not come from the aftermarket barrel, but from the custom fitting done by a talented gunsmith. He said it was the majority of the difference, and he used to work for Bar-Sto back in the day, so he knows custom barrels like very few others. So, regardless of the barrel you choose, make sure to get it custom fit from a smith who knows what they are doing -- don't just get a drop-in replacement that requires no fitting.
Oh -- post results! We want to hear how the old barrel does vs the new one!