So after trying it out for about 5 days, in simple words I would say that there's a slight improvement but I did not get the wow factor as mentioned by early reviewers and also the very convincing product description.
Of course, I still felt a bit of improvement, I did not have to press the pedal as hard as last time but it's just by a bit only. Perhaps I've set my expectations too high but nevertheless, it still gives a bit of improvement for me so I won't say that this product does not work.
Maybe bigger improvements can be felt on a completely stock car?
Anyway, I drive a MyVi 1.5.