Only true when you doing something really can utilise high sequential speed. For OS operation or loading apps. PCIe SSD won't do anything significantly better than SATA SSD (even with just the native SATA 2 port).
If you machine can boot in 20 second with the SATA SSD. Guess what? It can also only boot more or less in 20 seconds with the PCIe SSD.
The PCIe HyperX Predator can't even beat the Plextor M6 SATA SSD in the boot test. However, obviously all SSD require more or less the same time to boot.
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If PS can finish loading in 3 seconds with SATA SSD. PCIe SSD cannot make it finish loading in 2 seconds.
Because the bottleneck is not at the SATA 2 bandwidth, but the SSD's random read performance. Random read speed can't even saturate a SATA 2 connection. Give the SSD a PCIe SATA 3 card, or 4x PCIe connection won't improve anything at all.
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As you can see, the HyperX Predator 240GB M.2's random read speed only around 44MB/s. No where near the SATA 2 bandwidth limit. Therefore, M.2 or PCIe connection won't improve anything in the random read performance.
You may argue that the PCIe SSD still better than the SATA SSD a bit, however, not always true.
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In the above test, HyperX SATA SSD win the 4K read test, it's about 10% faster than the M.2
Also, please be remind that all these are synthetic benchmark. In real world, the difference should be even smaller because system cache will be enabled.
I understand that you simply compare that 1500MB/s (PCIe SSD sequential max on the cMP) and the 250MB/s (SATA 2 connection max sequential speed) to decide M.2 or PCIe SSD is the way to go. However, it really depends on your workflow. If I remember correctly, you mainly use this Mac for photos. That means.... lots of small files. So, I am quite confident that you won't benefit much by using a "faster" SSD.
If you want a M.2 / PCIe SSD, and you have the budget, go for it. However, don't expect it will give you any magic. They are just a tool, you have to use them correctly to release their true power. Using them in a wrong way is just wasting money.
If the M.2 cost the same as the other SATA SSD, then of course it make sense to buy the better SSD. However, there are plenty of low cost SATA SSD out there. If you ignore all of them simply because you believe that PCIe / M.2 SSD is way faster in normal daily ops, then you are on the wrong track.
