It partly depends on how many disks you really have; slapping them into an enclosure is a greener choice as you don't waste the disks, but for the cost you might do just as well getting a bunch of portable external 2.5" drives as these are pretty affordable now with good capacity, and should be bus-powered meaning you either plug them into the Mac Pro directly, or get a powered USB3 hub to run them from. If you can find a neat way to organise them then the cable clutter shouldn't be an issue.
I mean at the moment you can get a 1tb portable, bus powered 2.5" drive for £50 in the UK, or you can just get a 3.5", powered drive in a higher capacity. The things are so cheap now that buying an enclosure for old drives can end up costing a lot more for minimal gain.
This said; for USB3 you'd need at least four HDDs to risk saturating the connection, and even then only when doing something like striping them and loading a huge file. The main issue is that if you want to combine the disks together then separate drives may introduce latency issues, but single unit JBOD enclosures won't be any better in that regard.
Otherwise, if you're determined to use your existing drives then really you should go all out and get a proper RAID type system that can do performance and redundancy. The Drobo 5D is a popular option; some people complain about it but at least as many are perfectly happy with it, it's not cheap though, but it makes upgrading to larger drives or popping in extra drives nice and easy, and can do redundancy as well once you have enough drives to still support your desired capacity.
Personally I'm building a USB RAID enclosure myself; it's not that hard but can be fiddly. All you really need is a PC case with enough 5.25" drive bays to fit one or more hard-drive backplanes, add a PSU and a RAID controller and you've got yourself an enclosure. It can be a little bit more involved than that, but if you don't mind a self-build it's another option. You're still limited to USB3 speeds, and it's not going to save tons of money, but it gives you huge flexibility in the exact components you use, and for future upgradability. For example, I'm using a 9x 5.25" bay case, which will have two 5x 3.5" drive back-planes installed,occupying 6x 5.25" bays, this leaves room for another five drives in future, or maybe a much more compact back-plane for 2.5" drives. In this case I'm be using my existing drives initially, but adding a spare so I can have redundancy. With an on board RAID controller USB3 should be fast enough for my needs in general.
. Same price and it looks to have a large single fan so it might not be too noisy.
