That's really strange behavior, Brett. What version of ONTAP?
I don't have 10GB in my environment, so perhaps that's a different ballgame, but if anything I've been frustrated with the exact opposite behavior.
In my case, I have two 6000 series controllers at my DR site that are Snapmirror targets but do nothing outside that. In one case, I wanted to move a large, 6+ TB volume from one controller to the other. Long story short, because of the high cpu load on the source controller due to frequent snapmirror updates and lots of snapshots (deswizzling), I coudl not make SM push data any faster than 20 - 30 MBps on two GB links! It was being throttled because the filer was busy doing other things, and it didn't matter that I wanted the SM transfer to be top priority (there is no way for me to control that). In essence, I found Snapmirror did not alwasy perform the way I wanted because ONTAP throttles it so as not to overwhelm the controller.
Now I hear the story you just told...
Thanks for posting that. It's interesting to see that 10 GB either completely breaks ONTAP's SM throlltling or it at least does not throttle it very well, which to me is actually preferred. You can now manally control the speed at which it replicates.
One last thing to ask: Is this an async SM relationship?