I didn't see the 5th gear in question but most of 5th gear's "tests" are total rubbish because nobody behind the show appears to know anything about cars IMHO.
They did a back to back test of a load of "big" sports cars against the likes of Caterhams, Aerial Atoms, etc in one show I watched. I could have told them the results before they even bothered. Yes, of course a Caterham will whip a 911 on an autotest circuit and, surprise, surprise, a DB9 beats an Aerial Atom on a 2 mile drag down a long straight section of runway. A total waste of petrol doing tests like that. Now, put the Caterham against the Atom and it might have been less of a load of drivel.
Now, with petrol additives and octane boosters you will end up with less or the same power unless you recognise the correct circumstances to use them. They dilute the petrol with something that doesn't burn, at the most basic level, after all.
Put octane booster in a factory standard Corsa and it'll do nothing whatsoever. Get a race tuned engine with 12:1 compression and a tankful of supermarket petrol and it'll make the world of difference to how that engine runs. It will probably save it from being wrecked, in fact. If you don't start out with an understanding of what the product does, you won't test it in a meaningful way.
Kevin