Hey, maybe something I can help with:
I'm going through the reverse problem right now (writes faster than reads) but perhaps these suggestions will help you as well:
1) Jumbo frames (MTU 9000): make sure both machine's NICs are set to using an MTU of 9000. This opened up a bit of performance for me.
2) When performing a file copy between the two machines (both directions), run "top -aSH" from the FreeNAS WebUI shell and keep an eye on "smbd" when it shows up. If it is pegging one core at 100%, you might be CPU limited (apparently smbd is a processor hungry pain at 10gb speeds)
3) Make sure, in your FreeNAS BIOS, that any settings related to CPU performance are set to max, especially if the previous testing shows a pegged CPU core. You might also benefit from turning off hyperthreading.
I hope any of that helps as it did for me. I am, however, running FreeNAS 11.3-RC1 so you're mileage may vary.