Ok - First - some of the posters and confusing RAM with the SSD (I guess because the OP called it Flash. But, we are talking the SSD here. RAM is a non-issue.
As far as the SSD goes. This poster on eBay says the MacBook Pro and Mac Pro are interchangeable:
http://www.ebay.com/itm/Apple-Samsu...?pt=US_Solid_State_Drives&hash=item19ec687c0d
And
This article isn't sure:
http://www.anandtech.com/show/7603/mac-pro-review-late-2013/12
So, unless someone tries it and checks out speeds we won't know for sure.
I expect in time OWC will have SSD modules for the Mac Pro, but they will be lots of dollars, as all their stuff is. Maybe Transcend, they seem to be jumping in to the MacBook Pro and MacBook air blade SSD market with kits that are less then OWCs and seem to be more complete.
Lou