The built in support for ENA is nice but what about the other instances such as C3, C4, D2, I2, R3, and M4 (excluding m4.16xlarge) where you have EN using Intel 82599 Virtual Function interface? Why not support it out of the box?
The latest Amazon Ubuntu 16.04 AMI comes with ixgbevf version 2.12.1-k, while the Enhanced Networking documentation [1] suggests installing version 2.16.4.