Going back to the original thread title, I have found that replacement panel air filters are available. The OE paper air filter part number is 5Q0129620B for the UK Cupra models which is the same as installed in VAG group 1.6 TDI,2.0TDI,1.8TSI and 2.0TSI 2013 onwards engines. Even though they are not as good as a CAI filter installation, though all Leons do have CAI air filter boxes as standard, but probably not as efficient as a performance ones, the only reason to change the standard filter is to save money in the long term through being able to clean the cotton/ foam filter thus you buy one filter for the life of the car. Any performance increase will be minimal due to the design of the original air box and not the paper air filter. So here are the part numbers from the more common air filter manufacturers;
K&N 33-3005 Pipercross PP1895 ITG WB427 BMC FB756/20 Jetex 292175