There's 3 things that will make your XD more accurate:
1. Better sights if you can't adjust yourself to the factory sights.
2. Finding ammo that it likes. The 3 XD's (2 .45's, one 4", one 5", and a 4" .40) that I've put the most ammo through all like Federal's American Eagle ammo and brass cased Blazer. Even better, roll your own. While I had good results with American Eagle, those three XD's sure like my reloads better.
3. More practice with it.
Short of doing those things I don't think you'll make an XD anymore accurate. No, I've never used an aftermarket barrel in an XD nor aftermarket sights. The sights on mine have all been adequate, FOR ME. And maybe a barrel might help, but if I wanted a gun for Bullseye competition it'd be a hammer fired single action. Good ammo and practice are the best things to spend money on to improve practical accuracy with an XD. Anything else is just for fun or minor improvements, but it's your money you're spending.