We have used the Lantronics units. They work OK but aren't optimal
A protocol aware unit is best and will be faster. A protocol aware when it has a complete packet so it can relay the packet through the other port. If you were writing the code don't you think you would wait for the end of a Modbus packet before sending in on as a Modbus/TCP packet? What defines the end of a Modbus packet? When the going the other way will the Ethernet to serial converter know about the Modbus packet's timing requirements?
I didn't see where the protocol was mention or even asked for. Doesn't anybody think it will make a difference?