I shot a 6.8 AR (LWRC carbine) a few days ago, back to back with a .223 AR and a M1A (.308). Recoil was 'crackier' than a regular .223. It reminds me exactly of 9mm vs .40S&W vs .45ACP. The M1A was more of a 'thud' recoil, the .223 was a 'snap'. As to the LWRC, I have nothing but praise: well made, great components, comfortable, and I love the round ears on the front folding sight (Troy) as they helped me center the sight picture and repeat my cheek meld to the stock.
The owner of the 6.8 was surprised that my 223 mags went right into his 6.8, and vice versa. We'd both marked our magazines so we had no problems, but I could EASILY see the wrong magazine being inserted if they were mixed.
A tip for everyone: do NOT put a 10 round magazine into a FN FS2000. I am not happy with its performance at the range, as the 10 rounder got jammed in HARD. Also, I've been having feed problems with it. Specifically, it won't strip a new round out of the magazine sometimes. On the other hand, we both liked the Trijicon ACOG I had mounted on it.
Back to the topic, both 6.8SPC and 6.5 Grendel will work with a standard AR lower. Yes, both require a different barrel, bolt, and maybe some other stuff (gas system mod?), as well as modified 223 AR magazines. I would just buy a whole new upper designed for the bigger round then try to upgrade just some of the parts. Then, you can swap calibers back and forth whenever you want. Doing this would almost being like using 38 special and 357 magnum in the same revolver. You can practice with (inexpensive) 223, and then swap to the bigger round for home defense or whatever.