Hardware: Intel i7, 16 GB, Ati Radeon HD 7800
Software: Windows 7 Prof SP1, 64bit, Catalyst-Version 15,7 AMD Catalyst Omega Software (latest driver).
If GPU support is active Lightroom freeze under Developping mode. Photoshop deactivate the support with warning.
GPU support works when I reinstall the older Catalyst-Version 14.12 AMD Catalyst Omega Software
GPU support isn't ready for prime time. It only works on a few video cards correctly. Even if LR says your card is Good To Go 99% of the time LR runs better with that option turned off. So Turn it Off.