There is overhead introduced by the network, and the more machines you add, the more that overhead will be (depending on your network - there are threads discussing setting up fast routers and things).
Even with your network performing top notch and traffic not clogging it up, there's still some overhead in sending the scene and assets to the nodes, so it will never be 100% of the power (of course, that initial move of data will be less and less of an issue the longer a render runs, since it's just a one time thing). I don't have cold, hard, numbers, but maybe expecting 90 to 95% of the performance is reasonable - it will be close to 100%, but not exactly there :)
Also remember that nodes only contribute in final renders, not in Interactive Rendering (which is where the "one machine with the same Cinebench score as the multiple machines" will be a better choice).
And also, be sure you have a FairSaaS license that supports that number of nodes. The default is just 3 render nodes (but you can upgrade to more).
