Just wondering whether the two ports on the rMBP are sharing the same bandwidth.
If so, does the single-port, non-retina MBP have the same bandwidth with just one port instead of two?
Each physical Thunderbolt port can handle two (duplex) 10 Gb/s channels. Per Anandtech's very thorough review, the Retina MacBook Pro has four channels, two for each port.
A further Anandtech article says that the 2011 MacBook Pros used the Light Ridge controller, which has four channels, however due to having only one physical port, only two of those are actually available to the user. It's not clear what chip the 2012 non-Retina MBPs use, but it's likely the same Cactus Ridge chip used in the Retina machine, but with the similar limitation - each port is limited to two channels, so that's all you'll get, regardless of what the controller behind it supports.