Like you, i'm an 11 street shoe. I wound up squeezed into a pair of S-LTD's in a 26.5 earlier this season. Not the right boot for me. Partially because of the small size (race fit), but mostly because of the stiffness of the boot.
That being said, you definitely don't want anything larger than your foot measures. It took a few days to pack out the liners, but once packed out, they were actually quite comfortable while riding. I'd suggest a 27 -27.5, whatever you can comfortably fit into.
Greg