Yeah, that sounds like the right tack to me. As @Blaisorblade points out, the characteristics aren’t identical, and folks have established expectations of ArrayBuffer, so replacing the implementation with an entirely different data structure seems a bit iffy to me – not least, because a circular buffer isn’t what I expect an implementation of “array buffer” to be.
But adding it as a new data structure seems totally uncontroversial, and potentially quite useful. And making it the default mutable Seq is an intriguing idea, although I can’t say I’ve got a handle on all the ramifications there…