Matt, you're going to reduce bandwidth, but likely no more than when simply adding a new client to your primary network - at least while that connection is in use.

There's a ton of information out there about setting up an Apple device and other dual-band devices, including discussions on Apple's own forums and knowledge base. Surprisingly in fact. Unsurprisingly, I haven't been able to locate actual benchmarks yet, but i haven't looked hard either.

I've only looked briefly, but so far I haven't seen anything to indicate that bandwidth is lost when a guest is not currently connected.