The new 13ʺ Retina MacBook Pros have 2 Thunderbolt ports, and Apple appears to indicate you can plug in and use 2 Thunderbolt displays with them, but is the video performance satisfactory when doing so? I'm not planning on doing anything that graphically intensive, but I wanted to make sure that the performance was still fine. (Also, I'll be using the MBP in clamshell mode, so I'll only be driving 2 TB displays, not 2 TB displays and the laptop screen.)
Yes - the CPU/GPU are powered up enough for two external screens in my experience.
My experience is Apple won't bless as supported configurations where the majority of users will see laggy performance that cannot be tuned if needed with a firmware update or better drivers.
The higher end 15 inch retina MBP can crank out many more pixels, but the situation you mention with two TB displays with the Mac operating in closed-clamshell mode actually is less pixels to drive than the situation of one display and the internal display still running.
I run a 15" Macbook Pro Retina with two thunderbolt displays in clamshell mode. I have them daisy chained one to the other, then to the laptop. This configuration allows me to only plug/unplug one cable for all peripherals. I have had no lagging, video rendering trouble or over-heating.
I run them at native resolutions and it looks beautiful. I occasionally use graphics and video editing and playback software without reservation or degradation. I also have VM's running that show no lag.
I don't have statistics or links to back this up, only experience.
* EDIT - Adding screen shots of configuration *
If it's Apple Thunderbolt Display with a resolution of 2560x1440, I would say it might be quite disappointing.
My 15" MacBook Pro has a GT 650M, which is much more capable than the built-in Intel graphics. However, driving 3 1080p displays has given it quite some hard time, the UI is not as responsive.
It will work, won't be very disappointing, but doesn't live up to Apple standards.
Also, what application you run will affect you performance, what will you run?