Calaudude,
Actually, my recommendation is for the HP Z Turbo Drive.
I didn't try the Z Turbo on the z420, but the Z did make a better Passmark Disk number on the z620 than the bare drive did on the z420. Also, the standard SM951 is reported to be somewhat hot-running, so the Z Drive heatsink is welcome, and the bare drive price plus adapter cost about $30 more.
The purchase though was as a "new other"- a never used drive taken from a z230 and the useful feature was that it includes the HP recovery partition. The moment the system started, the window appeared: "Starting your computer for the first time." It had Windows pre-installed and since the z620 has OEM Windows 7 Prol'l, the installed version was up and running in the first 30 seconds, automatically activated and all the HP Performance and Diagnistic tools work. It was very nice too that the drive was setup as GPT instead of MBR- a safer format and more versatile partitioning configuration. It strangly appears in Control Panel /System as a z230, but other than a couple of error messages, "Windows ME wireless update unsucceccful" - what ever that is- it's performaned perfectly.
So, that was a pleasant shortcut and the Z Turbo cost $150 as compared to $157 for the Base SM951 + the adapter- $27 including shipping.
Since I bought the SM951, the supply dropped and the price increased: currently at Amazon - they're $185 so that would cost about $205 now.
If you use an M.2, remember to update the BIOS and chipset driver.
Cheers,
BambiBoom