Yes, GPU acceleration is currently available in Develop only. It is used during interactive adjustment of images for the main preview image. The benefits are more noticeable on higher-resolution screens -- such as 4K and 5K displays, and high-res laptop screens such as the MacBook Pro Retina 15" model (which has a 5 MP display).
There were some speed improvements made to Library as well, but these are not GPU-based.
Hardware configuration:
MacPro (late 2013)
AMD FirePro D300 2048 MB
Apple Cinema Display 1920 x 1200
16 GB RAM
1 TB SSD
OS X (10.9.5)
Test file: Nikon D800 NEF, 50 MB
(0) open the Develop module
(1) select a different NEF file and zoom to 1:1
(2) clear the ACR cache
(3) select the test file
(4) take 3 screenshots to illustrate the 3 display states (the first one is hard to capture)
(5) select another image
(6) same 3 states are present
(7) return to the test file and the same 3 display states are present
Why isn’t the ACR cache coming into play in step 7?
If I repeat this process with the GPU disabled the image is displayed without the intermediate states.
I have attached the 3 screenshots mentioned in step (4).