Will I Lose SSD Speed with Upgrade - 2015 MBPr

I just picked up an Early 2015 MBPr 13". Went down from an i7 mid-2014 MBPr 15" for the portability of the i5 13", but day to day I think this is actually faster. I would of gotten 2016, but I wasn't feeling the USB-C change just yet.

Anyways, the one I picked up is a 256gb and I want to up it to 512gb or 1tb of internal storage. I'm looking on eBay and It says the original Apple OEM SSD works in MBPr's from 2013 thru 2015.

My current speed from Blackmagic on the 2015 is "Write 1118 - Read 1425"--loving it!! The 2013 read and write speeds were way under that at like 700 and 300 read and write. If the read and write speeds were so much slower in 2013 (and also my 2014) How can same SSD work for all three?

Another question, and this may be a really stupid one. lol Will I lose speed if I am sent a drive that was salvaged out of a 2013 or 2014 machine? When I bought this, the Apple employee told me that with a bigger drive you actually get the faster the speeds, but this is obviously not what he was talking about. I still bought the 256gb and figured I could do it myself later if I felt I needed it. Two days later, I need it. haha

I'm obviously worried that I'll get a 2013 drive (not a 2015) and I'll lose that glorious speed.

Third question. I've read that before 2015, the flash drives were "2 lane" and the 2015 drives are "4 lane". I have no idea what that means, but If a "2 lane" 2013 drive works in my machine but is slower, how can I tell with the naked eye that an SSD is two or four lanes?

Thanks in advance for any help,

Dan