I run optimax in mine, without octane booster. It is recommended for 97+ RON fuel, but it will also run (poorly) on standfard unleaded.
I would look at there being other reasons for its' poor running as it is a UK car, so should be fine on most fuels.
Do you still have it under warranty - getting it looked at by a Subaru (or other) garage, plugged into a select monitor may show up some issues. Was it running the same before service? If not, get it back into them - it may be something pretty simple.
These are reliable cars - other than minor niggles, and major engine failures
Brian