Based on your evidence, Apple would probably like to know that they wasted huge amounts of R&D money on putting PCI-e SSD's in all their Mac computers then! You're only talking about BOOT TIME, 33 seconds out of an entire day of using a Mac. You're saying editing video or large Photoshop files is the same on an old SATA 2 SSD as it is on a PCI-e SSD?
I'm not sure why you are singling out "OS Operation", when my suggestion for an AHCI SSD was for better OVERALL performance? Do people really sit at their Mac all day booting it over and over again?
Because you mention about "clone the OS" to that drive.
I didn't say the high speed PCIe SSD is useless, but for OS, it perform more or less the same as SATA SSD.
Any SSD can turbo charge the Mac in daily use. And basically only large sequential read / write can utilise the PCIe SSD. I believe most normal computer users won't keep copying large files "entire day", but mainly use the desktop (OS stuff, e.g. Finder), Browsers, etc. Even graphic work and video editing not necessary can effectively utilise the PCIe SSD.
Photoshop is still very CPU single thread limiting on cMP, assuming the computer has enough RAM, a faster SSD won't help much. A 200MB photo is pretty large for a general user already, and it only require 0.8s to finish loading with SATA 2 connected SSD. A PCIe SSD can finish in 1.4s. Once loaded into the RAM, SSD speed is quite irrelevant. But is that half second really that significant for thew whole workflow? If a person only need 1 min to edit 1 photo (I think it's really very fast for a 200MB photo professional editing), then the overall time saving is less than 1%.
Video editing can be more demanding, but depends on how the user do it. If the user insist not the use proxy / optimised media to edit 10bit 4K60FPS video, then it may be a problem for SATA SSD. But if the user know how to optimised the workflow, reduce unnecessary pressure on the hardware. There is nothing wrong to use SATA 2 connected SSD for video editing. Before I get my SATA 3 card, I actually use a SATA 2 connected 840 Evo to do 4K video editing in FCPX. No problem at all. Even with a PCIe SSD, the most limiting factor during rendering still the GPU. And the most limiting factor during encoding still the CPU.
Apple was not wasting money on the R&D, but lots of people waste money on high speed SSD that they can rarely utilise. The actual gain they get from a PCIe SSD may be even less than 33s per day.
PCIe SSD of course have better overall performance, but do you know how expensive the AHCI SSD now? How much actual gain a normal user can get? Is it worth?
If anyone's main daily ops is copying large files back and forth, or (un)zipping for the whole day, or loading huge apps library for whole day (but not really using it), and cannot do this kind of stuff at the background. Then yes, PCIe SSD is very useful. But in this case, I will suggest he to buy a NVMe SSD, and use this NVMe SSD as data only drive. Why go for the AHCI that is slower, more expensive, harder to buy, and the only benefit over the NVMe is just "can boot natively" (and AHCI is not really faster than SATA SSD on boot / OS / apps loading).
IMO, there are 3 kind of storages now.
1) HDD, good for backup, very large storage, etc. Very cheap but slow.
2) SATA SSD, good for running OS (on the cMP). Cheap (compare to PCIe SSD) and fast (same speed as PCIe SSD for OS).
3) PCIe SSD, good for large files read / write. Very good as temporary storage for the current project to speed up the workflow, or act as scratch disk, or storing VM. Cannot natively boot in cMP, very fast, but very expensive.
Before we have NVMe SSD support, of course make sense to look for AHCI SSD (for demanding job). And if we have the AHCI SSD, then nothing wrong to put the OS on it (especially if the user do not have SATA SSD). But now, we have NVMe support natively, however, Apple didn't give us the NVMe boot ability. In this case, I believe use the cheap SATA SSD for OS, and then another NVMe SSD for work is a better choice. This combination can provide better overall performance than a single AHCI SSD, and cheaper than a single AHCI SSD, and may be more capacity than a single AHCI SSD as well.
High speed SSD can only turbo charge large files read / write. Everything else won't be speed up. It's very nice to have PCIe SSD on a cMP. But "install (or clone) the OS onto it" is definitely not the way to fully utilise it (if compare to SATA SSD).