I do some OSS development for UDOO, which sells iMX6 SBC (6Q, 6D, 6SX). We currently offer Android 6, I started to build 7.1 a few days ago.
Intro end, question is, did you compare the performance between Vivante and OSS stuff?
Work remains to be done with respect to Android. For other applications the OSS driver has been found competitive and even faster for some applications.
Unfortunately I have no numbers to back any of the above statements up.