low mpg

recently my mpg has been considerably lower than what it normally is...it used to be around 33 or so, slightly lower if i drove it hard. for the last week or two its dropped as low as 20 when being driven hard, the highest is around 27 when driving sensibly. i havnt changed anything on the car, i havnt changed the way i drive it (gear changes etc). the mods i have are a miltek decat and cat back and a pipercross air cone. any ideas what could be causing it? i know the weather is colder, but surely that wouldnt cause lower mpg especially once the engine has warmed up. a friend of mine suggested checking the spark plugs and cleaning the air cone to see if that helped, but i could do with some suggestions as to what could be causing this. i have really noticed a difference in fuel consumption, not only by the dash computer but also the fuel gauge, it just seems to be drinking it. also, i doubt its related, but recently both reversing lights have gone, and its not the bulbs or the fuse...thought this might be worth mentioning.

any help is much appreciated!

reversing lights :(

both of the reversing lights have recently gone on my st, noticed it tonight when i was reversing in the dark and couldnt see anything behind me. dont know much about cars, so was wondering if anyone could suggest what the problem might be and how to resolve it. dont really want to take the car into a garage and be charged a mega amount if the problem is something simple that i can do myself. seeing as both have gone i doubt its a case of blown bulbs, although as i say i dont know much about cars.

thanks in advance.