There'd be no point for having two weapons that be exactly the same. The BR's not overpowered. Sure it has a much faster kill time but that doesn't mean it isn't overpowered. Other traits counterbalance the BR's 4sk, including its poor accuracy, noticeable recoil, and how it has much greater flinch when shot. When scoped in, the BR can be one nasty dog to handle. It's hard to land multiple consecutive hits on an enemy from anything beyond close-medium range. I'm a BR lover, but even I can't use it all the time. Much of the time it's smart for me to substitute the BR with something else. For instance, if the team needs a support sniper to watch over them during a full frontal assault, it's probably best that I just use the LR instead because it has pinpoint accuracy and has an excellent killtime when scoped, but at close range it does poor due to its lack of consistency and its slow kill time when unscoped.
Two weapons with the same exact kill time wouldn't be balanced, because one of the two rifles could still have superior range to the other. An overpowered weapon should almost (if not) always be used over the others, because it happens to be superior no matter what situation you use it in. The Rocket Launcher, for instance, is highly overpowered to many other weapons like the BR (though it isn't much of a problem in Halo 4, because of its rarity, however I do still feel that the rocket launcher needs a small, flashing, yellow warning light to make the wielder a somewhat bigger target). Another more common example is how the DMR had a slightly longer kill time on top of much greater accuracy when compared to the BR. If all weapons are balanced, they should each be used in their own situation or niche.
Ever played Quake 3 Arena? That's how it is weapon-wise (excluding the BFG 10k, but that almost never appears in an online match except for a select few arenas).