I should've asked what's the best M.2 drive Asus Z97 Pro Gamer can use. This system is pretty close to what you got, as the CPU is i7-4770K with 32GB DDR3.Depends on the specific board and BIOS version.
Some of them can use a PCIe 3.0 x4.
Some, only PCIe 2.0 x2.
Some can boot from a drive in the M.2 port, some cannot.
But, the Z97 boards were released on the cusp of NVMe drives appearing. Support is shaky.
And as evidenced in the above videos...we are deep into diminishing returns.
I have an Intel 660p in a PCIe slot on my Z97 board.
Along with a selection of SATA III drives of varying ages and sizes. (list below)
In theory, the sequential speed of the 660p is almost 4x the SAYA III drives.
In practice, I literally cannot tell. And not just 'feel', but rather timed controlled tests.
My typical use of Adobe Lightroom...zero difference.
With the upgrade, I was hoping to get a faster typical usage, e.g. faster start-up, apps loading, games loading. From several reviews, I can see Samsung 980 can hold its own in real-world usage, even though it trailed behind in synthetic benchmarks.
My current system is i5-4590 32GB DDR3 with Crucial MX100 256GB. The boot time to desktop is 43seconds. It's one of the reason that made me consider M.2.