Attempts via REST:
Currently running on Confluence 7.3.2, it seems space permissions can't be expanded via
/rest/api/space/<SPACE_KEY>?expand=permissions
compared to Confluence Cloud where it yields permissions.
Attempts via SOAP:
The SOAP endpoints yields different XML on 7.3.2 compared to another 5.7 version I tested. I didn't manage to generate bindings (in C#) using the 7.3.2 WSDL.
On 5.7:

On 7.3.2
