The problem that your hitting is that Fortigate's Marketing department got a hold of the webpage for the product and gave you a useless as crap number (data rate_. What you need to know is the products throughput in packets per second (PPS). Unfortunately Managers like units they've seen before, so they don't feel "out of the loop". The size of the packet has almost nothing to do with the product's ability to analyze and forward it.
Almost any router, switch, or OS should be able to quickly tell you how many packets your sending and/or receiving. This is the number you'll need to compare against the different models. If you contact Fortinet's sales department they should be able to give you approximate values for their products throughput (in the useful PPS number). They may call it a "Packet Forward Rate", depending on what century they got into the biz.