Specifically on the processor it may be cheaper to get the base and replace it immediately for £1800 [1] than to buy the top of the line than to pay Apple the £2800 extra it seems to ask for what appears to be the same processor (cache, speed at least). And you might be able to sell the original.
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.
[1] http://www.scan.co.uk/products/intel-xeon-e5-2697-v2-s-2011-...