Never had one on a disco but had one on another car and there are cheaper ways of cleaning then but tbh if your not converting anything else I wouldn't worry although they can aid fuel consumption if they are kept clean
hello local member
as for the k&N there are reports of them letting fine dust past and causing engine damage they are expensive as sad they are cleanable but the oil cost more than a new paper type filter and they cannot be used if you have a car with a MAF sensor (v8, edc 300s, and td5) as the oil on the filter covers the eliment in the MAF sensors