Identifying bottlenecks
OP I think you're looking at the problem a little backwards. Your weakest link will inherently be the I/O bus.
That being said, raid-0 on PCI-e SSDs will be quite performant.

You need to ask yourself a couple questions


What are you trying to optimize for? Its a mac pro and you're cosnidering a significant $ investment, so you're probably doing some professional work. Keep in mind though that you're sinking a lot of money into a 5+ year old machine, and the parts won't be reusable on any new mac pro you buy in the future
What are your pain points now? You just ordered this machine, so you haven't actually used it yet as part of your workflow. Firstly, you might find it more than adequate for what you want to do. But currently, what are the things you are struggling with? We can help answer from there.