While AWS ES can be cheaper in some configurations, Elastic Cloud is actually quite competitive in pricing for larger clusters when compared to AWS' ES-service. This post compares the two services, and there's an example price comparison at the end of the post: https://www.elastic.co/blog/hosted-elasticsearch-services-ro...
We support most official plugins, and if you get a gold or platinum subscription you can upload your own plugins. Elastic's X-Pack is included in every cluster, which includes security features like role based access control.
It's not possible for external service providers to integrate with IAM at this point.
Or, alternatively, am I mistaken about how configuration works?
(A bit of history: When Found (the company Elastic acquired and which is now Elastic Cloud) was in private beta in early 2012, we actually did allow custom cluster topologies. We ultimately disabled that as it was overwhelmingly used to make sub-optimal cluster configurations, such as 5 x 1GiB memory nodes)