We currently have around a dozen composite projects, which all use the same set of API. The reason we use so much projects instead of a single big one is for performance reasons. When an API change ...
We're talking about a couple dozen APIs (we are using ReadyAPI to test a system with lots of applications interacting with each other, and each application have some REST API allowing us to check its state and control it).
But the performance problems are not due to the number of API, as far as I know. They are apparently due to the size of our projects (see internal defect SOAP-4680); But we also get performance problems on smaller projects (I'll send you a private message with the URL to a video I made for your support team regarding another issue, so you'll see by yourself).