I'm guessing its actual capabilities are basically just USB 2.0. Someone tore apart a Lightning-to-USB sync cable ages ago and the data pins are apparently wired straight through - so we know that Lightning can speak it natively - and given all the focus on simplicity and not doing multiplexing at the expense of more complexity in the adapter I can't see any reason why Apple would develop their own protocol.
That means that the CPU reconfigures the pins into USB2 mode, which means that Lightning can at least do 400Mbps. This is because any SoC nowadays can do low-level USB-device pinout, so it's reasonable to simply pass it through the lightning connector, unlike HDMI or DVI which require specific video encoders.
It says nothing about Lightning maximum speed. Besides, whatever maximum speed can be measured with today hw, it doesn't mean that can be pushed tomorrow with different HW in the iDevice.
Given there's nothing that can currently use more Lightning bandwidth than USB 2.0, and apparently 3rd-party manufacturers don't have access to anything except USB 2.0 and slow TTL serial, it'd be surprising if it did support anything else. Also quite expensive - Apple's hardware USB implementation is almost certainly third-party IP they've bought in and dropped into place unamended, modifying it to multiplex another protocol they don't need is a waste of money.
Besides, if we're talking about hypothetical future hardware, there's nothing to stop someone doing the same with micro-USB. (In fact, manufacturers already have in the form of MHL.)
