Not really an upgrade as these aren't really any better than anything Apple is currently offering as far as the CPU is concerned. In fact the more cores you have the slower the clock speed. A real upgrade in my opinion would be, say, 12 cores at 3.6Ghz or faster. I'm sure options like that are down the road. That's what I'm looking forward to. I'd probably buy somewhere else than OWC if they won't let me swap it out myself. It's not difficult. I cannot give up my computer for even one day.
12-core processors at the speed you're talking about aren't as sure a thing as you imagine- at least not in the near term. Intel has been fighting to for higher speeds in these multi-core arrangements for years- but keep hitting the heat envelope. Clock speeds have plateaued, even in quad-core implementations.
And you're argument about core speeds completely negates the benefits of parallel processing. Where optimization has been put into the software, the results speak for themselves.
I think the value with OWC is pretty clear- when you replace the CPU on your machine, I'm betting that negates your warranty. OWC service comes with a new warranty that you don't get if you do it yourself and things go bad.
You're right, there aren't going to be a ton of people who've bought a 4 or 6 core machine that are going to be looking to upgrade now, but give it a year, and if the apps that a person is using get better optimized then there could be a lot of 4 to 8 or 12 core converts.
As someone who has an 8core machine, I'm glad this exists because it mean then when the next generation Xeon's do come down the road, that there's an existing path for upgrading when the benefit is meaningful for me.