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 ...
I like this idea, it's something that I've been thinking about too. It's not just a performance thing, it's also an organizational thing--test suites are pretty flat in a single project, so it's clunky to put different APIs that each might have a number of test suites for themselves all into a single project when most of their tests are exclusive of each other. However, when those APIs do interact, either directly or indirectly, it would be useful to have access to the APIs from a central place instead of having to replicate and maintain them in each project.
If SoapUI projects had more nesting ability of test suits, then that would even be enough for me, rather than keeping projects separate. That would make keeping test suites for separate APIs in the same project easier to organize (the current test suite structure is too flat for this). Actually... I'll look to see if there's such an enhancement request and log one if there is one yet.
Edit: Just added the FR: