Hi Ian

This sounds like what Global Report restrictions is aimed at achieving .  Records can be restricted so they can only be viewed by a specified group of users. Note there is a substantial impact on report performance if you turn this on 

The restrict by organisation in the Filters tab allows you to select organisation framework and all children of this.

If the global report restrictions doesn't meet your needs then there is the Organisation Framework that can be added as one of the filters and there is also under the Content tab of the report the ability to show only the records of the user's organisation and the organisations below that.

You are correct that multi tenancy will be able to offer more separation of tenants in reports and other areas.

Regards