From Velonews...


"You can basically do that, with one caveat: The Shimano brakes will not work ideally with the SRAM levers.
Current Shimano calipers are built with higher leverage (longer lever arms) and are paired with a low-leverage lever that pulls more cable. So the SRAM lever, which has higher leverage and less cable pull than the Shimano lever, will not get the pads to the rim as quickly; they will need to be set up with the pads closer to the rim to get the same range of lever movement, and they will make the brake system more powerful than it was designed to be. That said, I used to have a bike set up with Dura-Ace ST7900 (10s) levers and SRAM Force calipers, and, once I got used to it, I had no problems whatsoever with it."

Read more at Technical FAQ: Shimano brakes with Sram eTap | VeloNews.com