There are at least two versions of the i.MX6 Dual. The "DualLite" version has no GPU for OpenGL ES at all. The regular "Dual" version does have the same GC2000 GPU as the Quad and it performs similarly in my testing.
OpenGL ES 3.0 will not perform any better than OpenGL ES 2.0 unless you need some feature that is new to the 3.0 API specification. Vivante has made significant progress in improving their OpenGL ES drivers, so it's important to use the latest version regardless of whether you need 2.0 or 3.0.
Regards, Clay