In my experience it actually helps quite a bit. Every transfer requires a few additional small writes to the sqlite databases as well as writing relatively small pieces to disk. All of that can be much faster with an SSD cache. You should see some improvements. That said, it’s definitely not worth it to buy an SSD cache for Storj alone.
Btw, I’m not sure which model you have, but I’m fairly certain you didn’t have to turn off the node or the Synology to add drives and then add the SSD cache. All of that should be possible on a hot system.
This may also be a good time to remove or rename your log file so you can start with a fresh log and measure performance with this new setup. Your success rate will probably go up since your setup and I believe your location is very similar to mine.