You can't overstabilize a bullet but the tighter the twist the more force the rifling takes as the bullet goes through. In theory this will wear out the rifling faster and create a lower muzzle velocity as the twist rate increases. I don't worry about those things as I'm sure I'll never put enough rounds through a barrel to ever worry about wearing one out and if I do then replacing a barrel is relatively cheap and easy. I don't think the velocity lost is going to be significant enough to worry about either.
What I do take into consideration is that since I want my ammo to feed out of an AR magazine I am limited to about 77 grain bullets. A 1:8 barrel is more than enough to stabilize the 77gr pills and I don't see the need to go any tighter so given a choice I would go with 1:8 but if all I had to pick from was 1:7 or 1:9 I would go with the 7 twist every time. Why limit yourself? This is less important with an XCR though as they are not known for great accuracy so chances are you won't be loading up a bunch of 77 grain bullets and shooting that rifle 500 yards.
The only thing you have to worry about with the 7 twist is if you are shooting very light and very thin jacketed varmint bullets at high velocity. Unless the bullet is built for the kind of RPM that it's going to have as it leaves the muzzle it's very possible it will come apart in the air shortly after clearing the muzzle.
My bolt action 223 is a 1:12 twist and I shoot 36 grain bullets through it and it does quite well with them but it's not suited well for long range shooting. If I ever re-barrel that rifle it will get a 1:7 barrel so I can start shooting long range with it and use 80+ grain bullets.
My AR is a 12 inch 1:8 twist and it shoots 77gr Black Hills match into sub moa groups at 100 yards. This actually sucks for me in a way because I didn't expect it to be able to shoot that well so now I will have to do a little load development for it and keep some heavier grain handloads in the stockpile for that rifle as well. I kinda preferred my AR to simply be fed bulk American Eagle and not worry about loading for it.
My non restricted ACR is also a 1:8 twist but I haven't found a handload that will do sub moa yet. Development continues whenever I have time to get out shooting
People that say it does not make a difference are wrong, it makes a huge difference but that difference depends on what you want your rifle to be able to do.
If you are only going to shoot 55gr Norinco garbage ammo then any twist rate will do since very few rifles out there will group that junk into less that 3 moa at 100yds.
If you want to shoot long range which would require a long (heavy) bullet then you will need a tighter twist barrel to stabilize that round at longer ranges.
The advantages of a 80 grain bullet over a 55 grain bullet are small at 100 yards other than as you mentioned a little more kinetic energy but the advantages are huge when you start shooting that 80 grain bullet beyond 500 yards. The increase in mass and ballistic coefficient allows it to travel further with less velocity loss, it doesn't get pushed around by the wind as easily, and it retains more energy further out than the 55 grain.
For use on an XCR you really can't go wrong with a 1:7, 1:8 or 1:9 twist, the rifle is not a sniper or DMR rifle, and chances are you won't be shooting it beyond 300 yards. 55 or 62 grain loads will probably be it's primary food and any of those barrels will be fine for that.
The best advice I can give you is that instead of buying a bunch of tacticool crap for the rifle and hanging junk off the rails you spend that money on ammo and shoot it more. Trigger time will make you a better shot more than any gizmo or fancy stock will.