At this time, Lookout cannot act as a Modbus slave over ethernet. The Modbus slave object only works for serial. If the PLC is what is containing the data, then you should be able to use the Modbus object to talk with the PLC and get the data from the PLC. The slave object is only needed if you want Lookout to act like a PLC on the network.
However, an update rate of less than 8 ms may not be feasible with the Modbus object. Unfortunately, you may have to buffer the data so that you do not lose any of it.