k-jones-087

ive had an ongoing boost prob with my bug wrx for ages,done all the usuall ovious checks,leaks,plugs,maf,etc. only code showing is for cat effeciency below threshhold p0420,quite common aparantly, so took down front cat to check but looked fine, checked rear 02 sensor on live data and was switching fast,so i really thought front cat was in trouble,until i checked emissions and there perfect. low idle CO is 0.08 and fast idle test @2700rpm is 0.14. im bit stumped, if my cat was bad why are emissions good?? any help would be great