civic nut

i have a 99 civic coupe 1.6 vti, after ages trying to get after market exhausts, turns out only 1 company supplies them for this car in england, so got a stainless cat back made, which im told can increase bhp by up to 4, i will take the cat off which im told will increase bhp by 5-10% does any body know if this is true?
and finally down pipes which are better in your opinion 4-1 or 4-2-1 and any significant difference between the 2, as im not sure appreciate your opinion especially from anybody with the down pipes,
cheers