Yes, I have ordered these parts, and will be doing this upgrade soon for both a 2015 and 2017 27”. Will post before and after R/W results. Still on the fence of also replacing the HD with an SSD, as the 970 Evo will be 1TB size.No one has answered your question. The answer is it depends.
A single SATA III is limited by that spec no matter what the enclosure or how connected—exactly as you suspected.
You can get a RAID enclosure to approach or slightly surpass the internal speed with four NVMe PCIe 2 SSDs. Very expensive.
You haven't asked the right question, however.
q. What is the best way to increase speed and add storage to your 2015 iMac?
a. Open it up and replace the SSD on the PCIe buss.
Apple installed a PCIe 2 SSD into the PCIe 3x4 buss on the 2015 iMac. You can double the speed with up to 2TB storage on that buss by installing a 970 EVO (NVMe PCIe 3x4) SSD with an inexpensive pin-out adapter. Under $600 for the parts plus an hour labor.
This gives you the equivalent of a 2017 iMac 2TB SSD except that it has TB2 ports instead of TB3/USB-C
If you want, when it's open, you can install a SATA III drive up to 4TB on the other buss. Yes, you can achieve the same with an external but it's less expensive to just add it at the same time you're giving that 2015 its Wheaties.
Yes but...Can you expand on this? It's my understanding that the interface determines the absolute maximum transfer rate.
Mike, I think you have answered here my question in a thread I created earlier about adding a SATA III SSD to a Late 2015 27" iMac with a PCIe SSD already installed. It will be awesome if I could have the SSD added internally - much faster than through the USB port and way neater than having something hanging off the back of the iMac.If you want, when it's open, you can install a SATA III drive up to 4TB on the other buss. Yes, you can achieve the same with an external but it's less expensive to just add it at the same time you're giving that 2015 its Wheaties.