I think this is absolutely top-notch work from some hardcore DIYers. Jason Koebler of Motherboard writes:
The Mac Pro 4.1 and 5.1 are known in the community as the “cheese grater” Mac Pro towers. These are the last highly upgradeable and modifiable desktop computer that Apple sold before moving to the much-maligned “black trash can” design that is sold today and hasn’t changed significantly since 2013. Upgraded versions of the 4.1 and 5.1 are, in many cases, the fastest Apple computers you can buy today.
People are putting new CPUs, RAM, SSDs, and modern graphics cards in the cheese grater Macs that are, in many cases, superior to what you can buy from Apple today. The Facebook group Mac Pro Upgrade is filled with people scooping up old Mac Pros from eBay and Craigslist and modifying the hell out of them.
The sheer fact that you can spend less money buying an old Mac Pro shell from eBay — and buy off-the-shelf components and get a comparable machine is really something else.