Connecting TVS before or behind Ethernet transformer?
Interesting question.

In reality it probably does not matter much which side you put them on, however there are pros and cons to both.

Connector Side

PRO: As a general rule of thumb you want transient suppressors as close to the connector as you can so the voltage/current spike does not go very far on the PCB and has less chance to cross-couple into other, unprotected traces and devices via capacitive coupling. Being on the connector side, obviously fulfills this requirement.

CON 1: The suppressors are really there to protect your electronics. Beyond the transformer they are really not "directly" protecting much other than the transformer which probably does not care if there is a spike on the line. 

CON 2: If a spike is sufficient to destroy the TVS diode to a short, you now have a shorted communication cable.

CON 3: There is some question about whether the TVS devices significantly changes the impedance of the line. This could impact the bandwidth of your system.

Driver Side

PRO: Here the device IS actually protecting your sensitive devices.

CON 1: The spike energy is dissipated in your side of the interface. This can result in transients propagated into the ground system of your PCB.

CON 2: Careful routing has to be established to ensure the lines from the connector through the transformer are isolated from other signals up the TVS point.

CON 3: It is possible that a sufficiently nasty spike can short the insulation in the transformer resulting in a significant reduction in functionality.

Summary

From the above, it would appear connector side is the better protection method, but driver side is less intrusive on the functionality of the line.