Well, it really depends on the drive on the TB port, and if your Mac can compress/process the video as fast as the drive can store it.
But using two different drives, will make a difference.
But I guess the best solution is to store the 'scratch' file on the SSD and render to the TB drive..!
And then delete the temp file from the SSD when you are done..
Update: If you need to test your drive speed, AJA's System Test is a great little utility!