Your first point is not true compared to Ubuntu at least [0]. AWS is basically are repackaged version of KVM (the last time I checked), so there's no particular advantage. Ubuntu (and I'm sure Centos/RHEL) provide custom images which have the same optimised code.
[0] I'm pretty sure it's true for CentOS / RHEL, I just don't know for a fact.
AWS is mostly Xen (they just introduced a KVM option https://aws.amazon.com/blogs/aws/now-available-compute-inten... ), and there are a few things like "enhanced networking" where you get a performance boost if you're on a kernel with support but it's still functional if you're on any other kernel.
Also, and for many more importantly, it's probably easier to get support from Amazon if you're using their official Linux as opposed to a third party distribution.