1. It seems that the most logical and practical application is for transferring images from DSLRs - is this still the case?
Also for expanding internal storage with cards that sit flush, either just for most storage space or for backups.
2. I believe that the SD card slot is connected by PCIe, and SDXC cards (Class 10, UHS-3) reach speeds of 95MB/s - what sort of real life transfer speeds could one expect at this rate?
I don't own a Mac that is new enough to have a PCIe connected card slot, however
some people have reported speeds of around 80MB/s read with a UHS-3 Class 10. This only applies to modern Macs though as on older ones it is connected via USB.
3. Although they are crazy expensive, I have found 512GB cards - does anyone use cards this large or simply multiple 64GB cards, for example, which seem to be relatively good value?
If you are using it for something like storing a large iTunes library or for backing up then a 512GB card may be worth it, or if you simply don't want to have to swap cards regularly.
4. Is it feasible to run/boot Windows (whatever version) from an SDXC card?
Maybe, but from the internal PCIe connected SD card reader,
maybe not due to either Windows not being able to boot from the PCIe connected storage or the UEFI not supporting it anyway.
5. Can I compare the 95MB/s speed to, say, a typical 5400rpm HDD which may transfer at ~100MB/s or am I comparing apples and oranges?
You can compare them, however note that those speeds are for sequential transfers. So for example just reading/writing writing a single file.
A more important test is the random access which has a larger impact if you are running an OS off the drive, that said an average SD card I think will perform similarly to a hard drive in this respect as they aren't really designed for this.
Ultimately I'd just like to make more use out of an unused slot, but also wondering if it would make any sense to use spend $ on an SDXC solution, or say just spend the ~$200 on a 2TB internal HDD for my optibay?
If you have the option of an optibay then running an SSD and a HDD is a better solution, with your OS(s) on the SSD and media and other things on the HDD. Alternatively you can create a DIY Fusion Drive.
Though regardless if you don't need the optical drive, then this would be the best way to go, both price wise, ease of setting it up and getting it all working (if you want to install Windows on it) and likely performance wise as well.