Hello Gareth,
The K & N air filters will pass more air than a comparable pair of paper element filters, but the point that was being made is that the current location of your K & N filters means that they are exposed to the direct radiated heat from the exhaust manifolds.
An engine will always produce more power if the air being consumed is cold, but of course this is not always possible, so choosing a system that promotes a cooler air supply will be beneficial to engine performance.
If you purchase a pair of K & N filters that can be accomodated within the standard air box, then the air being drawn in will be cooler compared to your existing arrangement.
Ron.