The FNAR is built by FN to be a reliable & accurate LONGER range .308 semi-auto, based on the older Browning BAR HUNTING RIFLE ACTION.
It has nothing in common with an AR-10 design aside from the .308 caliber.
You can't switch calibers or "uppers", since there is no switchable "upper" on the gun.
It IS accurate at longer distances, with a good scope.
It's reliable, it's also heavy & a nuisance to break down for cleaning. Unless you hunt from a stand, you won't enjoy hunting with it much.
FN intends it for police & long range shooters, 500-800 yards or so, with good glass. It has no iron sights, as you probably noted, and whoever told you it's an AR10 was an idiot, IF it's an FNAR.
If what you have LOOKS like an AR in profile, has an upper & lower receiver section, and uses an AR mechanism internally, then it's not an FNAR.
I've never dealt with subsonics in the .308, but there's nothing wrong with a 1/12 twist for general use in the 168-grain range for the caliber.
You can't change the twist rate in the barrel, unless you go to a larger caliber. Not cost effective & probably not particularly compatible with the gas system & action as the FNAR is set up at the factory.
Your best bet to get what you want is sell the gun unfired, if it is an FNAR, take the loss if any, pay more attention to what you're doing BEFORE you buy next time, and get an actual .308 AR-10 from whoever.
Also, suppressors will still not make your gun "Hollywood quiet" even with subsonics, you may end up with more gunk in your AR action, and the suppressor will require cleaning & maintenance. You'll also lose distance & power with subsonics. You may want to re-think that, too.
Denis