I recently migrated a customer from Notes 8.5 to 8.5.3. The customer was a small shop so there backup solution was also very small. To help him to save some backup space I moved the "fulltext index" to another folder, which is excluded by the Backup software. This can be easily implemented.
All what must be done is to create a folder on the same server (I wouldn´t prefer to use a network share because of a potential bad performance). Then add the following notes.ini FTBasePath=d:\full_text entry and reboot the server.