I asked back in September about getting the Centos6-HVM AMI updated to support the C4-8XL instance type. Karanbir responded

https://lists.centos.org/pipermail/centos-virt/2016-September/005222.html

but I still see no evidence this has happened. This is a major thorn in our side and has prevented us from going completely to the C4 instance type ​, which we need for performance​ (both CPU and network).

Thanks in advance

John