After having used the XCR for a couple of weeks, I get the impression that every possible mod (except optics) actually hurts the performance of the rifle.
6.8mm barrel almost definitely lowers damage further than it says in the description; it is not uncommon that I find myself needing 6-7 body shots around ~20 yards to put somebody down. Bloom is manageable. The vaunted penetration is usually not enough to justify the very low stopping power.
The 7.62mm barrel DESTROYS Robinson's accuracy and gives it absurd bloom, to the point that even if you crouch and tap slowly with Iyan gloves, you will find yourself drifting very far from target with every shot, and it will become almost impossible to consistently engage and kill anyone beyond 20 yards.
ROF trigger exacerbates issues with bloom and accuracy on either barrel; Stability trigger with 7.62mm barrel gives the gun a ROF that feels in-game slightly higher than spraying HK417 on full auto. It's difficult to use optic mounts with either barrel, because of the kick and the extremely low ADS ROF.
However, I find that with no mods whatsoever, the XCR hits a sort of middle ground, with very controllable recoil, average recoil, sub-par bloom, decent accuracy, and seemingly higher stopping power than advertised on the stats. Doubtless, some will disagree with me; but this has been my experience so far with the rifle.