Have you looked at buildkite (http://buildkite.com)? It's a very different approach to CI - you use your own machines. That means you have access to your own s3 buckets, you can talk to your other services and get to use all the CPU and ram your machine has.

We switched over from Circle and it's been awesome to scale our build process without fear of resource constraints. It's pretty great :). They even have a CF setup that will auto scale boxes based on CI load.