Speed difference between traditional SSD 2.5s and PCI Express Type: SSD - M.2

The NVMe / PCIe interface is faster but Tosh don't say what SSD they use so impossible to know what the actual speed is.

==
Spec of SSD on Crucial ...
Specs: 1TB 2.5-inch internal SSD • SATA 6.0Gb/s • 560 MB/s Read 510 MB/s Writ
==

600 megs a seconds .. Is the speed Im told by lenovo sales

so what the % speed difference with PCI versus sata cable?

The most important factor is the actual media and controller used for both drives. You can't say which one is faster simply by looking at the interface used.

Im considering getting one of these PCs  ...

https://www.westcoast.co.uk/Brands/Lenovo/Workstations/30BX004JUKLenovoThinkStationP520c30BX.html

Pulling out the drive and adding a 1TB one of These
https://eu.crucial.com/eur/en/ct1000mx500ssd1

Or adding it in as additional storage

I prefer having one drive, Ive never used a smaller C for op sy and a D: for storage -  but It can lead to issues Id imagine, its not ideal

OS on C and data on D is standard practice for servers, not common for PCs though.

The crucial has power loss protection and a variable size pseudo SLC buffer so if there's enough space free it can expand that buffer for large writes. Without a SLC buffer short writes can be fast but slow down as TLC takes longer to write to than SLC. Again we don't know the model of SSD the laptop comes with so we can't even tell if it is SLC or QLC, QLC would give worse write performance for large files after any buffer is used up.

I've often found it better to have a separate data drive, even on a PC.  This keeps the system SSD with more free space, so the wear leveling has more blocks to move to, and they will last much longer.  Even when I had spinning disks, it meant that a reinstall would not require that the data be wiped from the disk, requiring a backup, then a recovery.

I'm still waiting for a time when Windows can more easily do a system recovery without wiping/affecting the data, applications and preferences just like a Mac does to fix/replace corrupt system files.  The current Windows recovery process is much more tedious to do.

Not going to get into the specifics of that machine.

SATA is limited to 600 MB/Sec.  Any drive, regardless of what form it comes in (looking like a typical hard drive or one that use an M.2 connection), if it's SATA will not be faster than 600MB/Sec.

NVMe drives ALSO use m.2 connections and connect, typically, with an x4 PCIe connection.  This is MUCH faster.  While there can be variations, IN GENERAL, with a modern system supporting m.2 connections directly, you can expect the NVMe drives to be roughly 5x faster than SATA's maximum.  

Put another way - and this is a broad generalization:
if the relative speed of a spinning 7200 RPM hard drive connected via SATA is "1x fast", then an SSD connecting via SATA is "5x faster" and an NVMe drive connected directly to the PCIe bus is "25x faster" than a typical spinning drive.

When I go to Toshiba's site and select the ThinkStation P520c and pick customise it lets me add 1024GB Solid State Drive, M.2, PCIe, Opal, TLC for £520, set as primary and then remove the original, so they do sell a 1TB variant.

1970 VW beetle 81 MPH max.

Here's a NVMe drive with horrible sustained write performance once it's buffer is full.
https://www.tomshardware.com/uk/reviews/intel-ssd-660p-qlc-nvme,5719.html (half way down page 3)