I'm curious about peeps opinion on this too. I've tried the 13" with Apple's Cinema Display combo at two separate stores now. One was considerably better at handling the ACD for mission control and web browsing which is strange. Both however showed a noticeable drop in frame rate while pushing photoshop full screen on the ACD. Very disappointing for me. I'd like to try the 15" with and without the extra V card but both stores have refused setting them up for me.
I have the 13/2.8 i7/16GB/1TB and no lag anywhere in regular use, or during any of the transition animations like mission control, whether using my cinema display or the native display, or both simultaneously
I sold off my 27" ACD 2 days ago after switching to a 13" rMBP. The ACD was connected to a mid 2013 13" MBP with 8GB / 120GB SSD.
If you plan to connect an ATD or ACD, i suggest to evaluate the visual / UI performance with a closed lid and leveraging the Bluetooth or USB input devices.
Although the 13" rMBP is capable of driving 2 ATD, the user interface will suffer because of all the pixels it will push.
I connected my rMBP to my ACD to test it out (with closed lid) and it worked more fluid than the scaled resolution that I prefer on the rMBP (1680x1050).
If you want to have the most fluidity, buy a rMBP with a discrete graphics card or an iMac. Integrated graphics will always be a bit more sluggish but I personally don't really care as I spend time using the computer and not watching UI animations.