MacOS – Should I create a Fusion Drive before or after downgrade (from Yosemite to Mavericks)

You can use the command-line iostat tool to monitor disk activity on a per-drive basis.

Runnning iostat disk1 disk2 1 will give you an output of disk activity for disk1 and disk2 every second. You'll want to change the disk identifiers to match those of your HDD and SSD.

By running this command while you write a bunch of data to your drive (the dd command can be useful for this), you can watch which drives the writes are hitting. Once you fill up your SSD, it should start writing to the HDD, and once the writes are done, you should see some activity transferring ~4 GB off the SSD to the HDD.

You can also test whether data is promoted off the HDD by reading some data off the HDD (again, dd is useful for this). Depending on how much use your Fusion drive has already had, you may need to repeatedly read the same data in order to get it promoted, but once it happens, you should see some transfer activity, once you're done all your reading.

For some more detailed descriptions of how and what to look for on a DIY Fusion drive, check out the latest posts on this Tumblr.

From my hands-on experience, I'd have to say, "negative".

I've created a couple of fusion drives on a 2008 Mac Pro from devices with preexisting partitions. Building the fusion in Core Storage destroyed all partitions on the media.