Yes they can. Think of your engine as a big air pump (because that's all it really is). It's pumping air in & out every minute it's running & the easier the intake path allows the air to flow in, (or out, look at the exhaust side too) , the less energy the engine wastes trying to suck the air through a restrictive intake. But it's like you say, you have to be able to keep your foot out of it to realize the MPG gains 
something that's hard for most of us to do at first lol
something that's hard for most of us to do at first lol
