Mac Pro 5.1 (2012) memory upgrade

NO.

The memory controller in the MacBooks with upgradeable memory is very simple, and can NOT downgrade speed.

Users who install memory faster than the EXACT correct speed ratings have their MacBook Pro become flaky. They crash about once or twice a day, and for a different reason each time.

Users who install memory slower than the EXACT correct specs have MacBook Pro that either do not boot at all, or do not stay running for more than a few minutes.

In the US, this vendor:

http://MacSales.com (OWC) is still the most-recommended supplier here, because they have good return policies and a lab full of Macs. They pride themselves on their service, so they do not sell stuff that is so "difficult" it needs a lot of hand-holding to get working.

OWC sells their own private-label SSD drives, which are pretty good and reasonably priced, and you know they will work in your Mac.

Look for the "My Upgrades" link on their main page, and it will lead you to parts that will work in your specific model Mac.

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Appropriate Vendors have a lookup program where you enter the specs of your computer, and they recommend the parts that will work under these terms:

"It works in your Mac,

or your money Back."          (it costs them money to get it wrong, so they make certain they get it right.)

Also, a Lifetime Warranty is standard, and their added vendor stickers on the modules are all you need for proof of purchase if you ever need a return.