From a quick research, this is what I can see:
I'm going to assume that you're wanting to know about transfer speeds of external drives. Your internal drive (20gbps) can be as fast as 1500GB/s read/write, or it might be a bit slower, like half of that, depending on the size of the drive.
I don't know, but I don't think you can swap the internal drive to get a higher speed. I think the speed is determined by the speed of the PCIe bus.
You have 1 Thunderbolt port that can (unless you're using it for an external display) deliver 20gbps (Gb, GigaBITS, not GB, gigaBYTES), that means up to 1500GB/s often less.
Then you have a couple of USB 3.0 ports (5gbps), and in my experience that gives me around 300GB/s r/w speed, maybe a little more.
For context; 300MB/s is really not bad at all. Remember spinning HDs? The fastest ones with 7200rpm had 70MB/s r/w or lower.
Do you find the speed of the internal drive too slow? What are you hoping to achieve?
Here's a free app to measure disk speed with:
Download Blackmagic Disk Speed Test by Blackmagic Design Inc on the App Store. See screenshots, ratings and reviews, user tips, and more games like Blackmagic…
apps.apple.com