Was there not a part on fifth gear years ago about these and octane boosters . Iirc it was just a waste of time and money and much the same with most of the octane boosters .
A long time main dealer Subaru mechanic who has looked after and serviced every type of subaru said that they fitted them to a lot of cars over the years and are good for 6-8k and never had any issues with them so just said I would see if ye had any experience with them.
Had one on the 05 Wrx fitted by finnegans. The hose pipe frayed going to the thing and sprayed petrol all over a hot engine. Disconnected it and didn't notice any difference