Here are a few points I managed to gather on compression:
Mode 3 is the best but it only tolerates up to 2.5 Gbps of FC traffic;
We're using 2 here (moderate) and we get a ratio a bit higher than 4
It is a disruptive process as by the time you change the parameter on both side, it will raise an "incompatible configuration" alert and turn the tunnel down.
Might be worse before making the change to do an "ipperf fciptunnel -tperf" with different compression parameter and see what you get. WARNING : activating tperf on the tunnel turns the tunnel down for the same reason than compression!
Have a look also at what is your peak data send rate on your line to see if you can go for mode 3 compression.