end of the day common sense tells you a k&n is just as good, and so is a £10 ebay filter, but thats not the point is it.
That's true, the actual filter is probably similar or the same in GSR kits as it is in K&N or cheap eBay filters, but the GSR kits are positioned in the engine bay to make the most of the cold air coming in, for example the 1.9 JTD induction kit sits right at the bottom of the engine bay.
Also, at least with the GSR kits you know what power gains you are going to be getting as they have been tested on a rolling road before and after having them fitted, whereas with a K&N or similar induction kit the company makes claims that they haven't proven.