They make a difference but not so much to the performance. Its a worth while mod as my my brothers C20XE kicked oil into the inlet and made a mess. He didnt want this happening to his new re-profiled inlet so got a catch tank. It doesnt kick as much smoke out of the back either now either.
Would never use a filter alone due to the mess it makes. Use one with a catch tank to vent the top but be warned- it will make a smell and my brothers kicks a bit of smoke out especially when first starting (it visable)