Is there any published information (by the community or Microsoft) about performance of the new web service based Managed Client Object Model versus the pre-2010 object model?
No official statement AFAIK, but the client object model will always be "slower".
The Client Object Model calls a web service (client.svc) which uses the SP2010 object model to return the result.
It is most likely faster than the old web services, since it only transfers the requested data instead of XLOBs (XML Large OBjects :-). JSON is used over the wire.