If I'm going to drop $5000 on a computer that's not meant to be upgraded like that, I'm not going to immediately take it all the way apart and risk ruining it to save $1400. I would either pay it or not buy the upgrade.
I wouldn't either (would probably just go for the cheaper one if I was going Mac Pro which I am not) and the 8-core might be the better upgrade anyway looking at the Anandtech review.