TurboQAM support was introduced in the controller-based AP-22x in AOS 6.4.2.0.
For Instant, support will likely appear in v.4.1.1.x this quarter.
Keep in mind that TurboQAM is a Broadcom-to-Broadcom feature and only works with TurboQAM enabled NICs. Be sure to check your NIC specs to see if it is supported.