I tried 32GB LRDIMMs in my 12-core late 2013 Mac Pro. It did not boot. I assume that the BIOS/EFI is not compatible.
I also tried 32GB HyperCloud DIMMs in my 12-core late 2013 Mac Pro and they boot just fine, running at 1333 MHz. Standard RDIMMs would run at 800 MHz.
However: They run very hot, and the fan starts spinning at full speed very soon after I start stress-testing the memory, even though CPU temperature remains low. Because they require a heatsink, they don't fit unless you take the Mac Pro apart and remove the DIMM mechanism. Even if you do that, you have to come up with a creative solution to keep them secured in place.
Overall, I wouldn't recommend them. If you need 128GB of faster memory on a Mac, get a cMP or build your own hackintosh. Otherwise go for the regular 128GB Mac-compatible memory, which runs at 1066 MHz or 800 MHz depending on your CPU.
I'll chime in.
I have a nMacPro with 128 Gb of RAM running at 800 MHz. While the memory is not running at 1.3 GHz I still think is great.
I run memory intensive tasks and with 128GB of RAM, storage is no longer the constraint, the CPU is. Usually memory is waiting for the CPU to finish calculations, even though more than 32 GB of RAM is used (thus needing at least 48GB or 64GB of RAM installed) the extra RAM then can be used to have other programs open even though they don't task the CPU.
With that in mind, 1.3 GHz Memory speed, buys me very little, the only applications that I can think of that can pay off is a database where stuff gets in and out of memory or changing memory locations constantly. I haven't seen any folks share here an experience using a Mac Pro for that. (I am sure there are some out there, I am just saying they are not in the forum)
I see most people here trying to hack old Mac Pros to get more value out of them, which is cool. But I will give you a very good reason that is not going to be popular here to get a New Mac Pro. Under load and fully equipped the Mac Pro is DEAD silent.
The general consensus here is that you can spend less and get some hacking with a PC or old Mac Pro to get it work. But I will tell you this. I used to have a 5.1 Mac Pro with 128Gb of RAM and 3.4 GHz processors and at full utilisation the Mac Pro will be making some noise, doesn't matter how much you slow down the fans with a hack. Since you are pushing the thermal profile of the mac with the CPUs AND the memory, it is a constant buzz that really is audible. With a PC is even worse.
With the new Mac Pro, I have used at 100% for days on end and it doesn't make an annoying noise, the loudest thing on my desktop are external hard drives, by far. I could very well sleep right next to it. If you get a hackintosh, or as I said an old Mac Pro, there is NO WAY you can get the same quietness. Again, I know most people in this forum dismiss this, but once you are on a desk for more than 8 hrs, or worse you have it in your bedroom at home and working it at full speed, that feature alone is worth the extra price to me.
If you are strapped for cash and you have literally no other concerns (time, noise, design, size, etc, etc) then yeah, get a PC and put the memory there and share it or use it as a server.