It probably used the basic zips compression like 90% of all EXR implementations. Not sure, though. I prefer using TIFFs or PSDs directly for working on 3D rendered images, the rest I do elsewhere...
Previously Photoshop always used the wavelet compressor for EXR.
And Zlib can be very slow, and very data dependent.
It's safe to say that PsCC doesn't have full support for EXR or partial support for EXR 2.0 ?