Ok, so I deleted Roon and all stored data from my iPad Pro & iPhone 7
It actually took a while to process the deletion and for a while you could watch as the data was still being deleted from the ‘Other’ category within the storage usage display.
Once storage usage stabilised, cleared all running applications, rebooted the device and then re-installed Roon and applied a few config settings to get it back.
Data usage by Roon is now 359.4MB, let’s see during usage over the next week or so whether this gets back up to the gigabytes.
However I still think there needs to be better storage caching and management in the iOS App.