First a caveat: The numbers I am quoting are from the spec. sheets.
Secondly: The throughput quoted from the spec. sheet (and identified in your posting) is for the current HW version. Older hardware is slower (see the overview published on the FAQ). To check what you have (if you have one in hand), see the hardware revision overview
Thirdly: The throughput is the aggregate - each ethernet port is limited by its own speed cap, which for a GE port would be 1 Gbps. If you have a single WAN connection then it would be limited to the port speed - 1 Gbps. If you have four WAN connections, each running at 750 Mbps for a total of three Gbps then I would expect the 1350 to be able to handle that.
Finally: This is all off the spec. sheet. Note the footnotes (e.g., “Router Throughput and PepVPN/SpeedFusion Throughput is based on Ethernet frame sizes of 1280 or 1518 bytes. Actual performance can vary across different networking environments. Figures apply to the latest hardware revision.”)
Wiser heads than mine may correct or elaborate on this 
Cheers,
Z