To my understanding SSD benefits mainly for Roon database operations, so if it works fine for you now it should continue even with more zones.
I was running Roon on QNAP TS-869PRO (quite old Intel Atom) with 3GB RAM and all was quite fine (tolerable for me) except search, which took like 10-15 seconds to provide output 
After I moved Roon to i7 NUC with NVME SSD it’s instant for any operation (except Roon start or database reindexing / metadata rescanning). I have around 7k albums though…