Not wishing to be left out of the graphics driver party, AMD has released a new 15.4 Catalyst beta with optimisations for GTA 5. You can download it here, and it specifically states that it “enables peak driver performance optimization for AMD Radeon™ graphics users in Grand Theft Auto V.”
15.4 adds a crossfire profile for GTA 5 (as well as other titles like Metal Gear Solid V: Ground Zeroes and Sleeping Dogs: Definitive Edition.) According to the release notes, it also fixes a couple of issues with Battlefield 4 and Battlefield: Hardline.
Make sure you grab the Catalyst 15.4 beta intended for the version of Windows you happen to be running. You might also want to remove all traces of the previous driver with a third-party program like Display Driver Uninstaller before installing the new ones. Doing that, of course, is at your own risk.
Nvidia users are covered for the upcoming release of GTA 5 on PC as well, with new GeForce drivers issued earlier in the day.