Yes, as a drive gets full it will slow down some, but it would need to be very very full for you to notice any difference. Like over 90-95% or so full.
The flash storage in the Retina is not soldered in and is upgradeable, but the problem is nobody yet makes a replacement for the 2013 models. OWC made an upgrade for the older models and I believe they have said they are working on an upgrade for the current model.
I don't know the answer to this question, but I was under the impression that hard drives got slower as you filled them up because the heads would have to move across the disk and start filling programs in the nooks and crannies that were free between other areas of the disk that were taken up, causing more head movement and thus more time seeking.
Not sure if SSDs have the same kind of problem or not, or perhaps it's just a thing that happens as the OS tries to cope with such little storage, not sure! Would love to see someone more educated on the subject chime in here.
SSDs' write speeds (especially random writes) decrease dramatically as usage approaches 85% full. This is because there are more and more filled blocks (NAND memory adds an extra "E" state to the traditional 0/1 block states of HDDs) and the SSD must first clear (reset to "E") and then write.
Now, the read speeds should be fairly consistent until every cell on the SSD has been written to at least once. For the most part, TRIM and garbage collection on modern SSDs and operating systems will alleviate these sorts of problems.