Exhausts requiring backpressure is a myth as old as cars, and it's been disproven.
I think both sides on this argument might be going to extremes. My personal experience with and faith in the engineering prowess of such companies as Borla and Corksport tells me that sticking to a 2" pipe might not be the best idea. But at the same time, a post I read awhile ago from someone who really seemed to know his stuff also makes me think that back pressure is indeed a real factor to be taken into account and that going with say, a 3" pipe or larger on our NA engines might not be a good idea. Here's the post by daygoVR6 from another Mazda forum:
"ok i gotta reply to this cause its really got me shaking my head...idk where people get their info from...I worked in the R&D department of JBA Headers and was behind many many projects..I also help tune a 2000+HP Turbo mustang drag car and have worked with multiple race teams building engines and cars designed to do one thing, put people back on their trailers crying like a bitch....so before anyone slams me keep that in mind cause Im not going to waste time trying to argue
High flow cats do not do much for small displacement, small hp engines/cars and are typically a race only modification - the exhaust will only get louder - exhaust companies/aftermarket companies like to market a product like a high flow cat for street cars simply do one thing..make more money...high flow cats are great for boosted applications that are making 300+hp to help reduce spool time by reducing back pressure, or high HP naturally aspirated engine applications that are on the street - typically the 400+ hp and up crowd
Sorry our small hp engines will not benefit much if at all - literally a 2-4 hp gain at most is what you will see and it will only be apparent on a dyno...if you are looking to gain 2-4 HP with a $200+ part ...go change your oil to full synthetic in the crankcase and trans...there is your 2-4 HP gain, and it will cost less initially...
A high flow cat will also reduce your back pressure too much in many cases on the smaller 2.0L engines and you will actually loose your bottom end torque VS a simple cat back exhaust system (ever heard of too much of a good thing)...meaning...you will be slower in your 0-60/70MPH accel. time ......you will gain some umph on the top end - so there is some gain to be had and this is not all negative -> but the added seat of the pants feel you wont get until you are around 75MPH and up - how many of us here actually track their car or get to where they are going 75+ on a drag strip...maybe the last 300 feet on the 1320 with these cars...and the gains for the seat of the pants feel is minimal at best. And keep in mind you are going to be street driving and not always under wide open throttle where the gains will be most beneficial i.e. like on a track
Changing a catalytic converter to something other than the factory part in ANY STATE is highly illegal and can incure some hefty hefty fines, yes a high flow cat is potentially passible visually, cause not every tech out there is that smart...but a lot of shops wont touch the car unless it comes in with no plates and on a trailer and looks to be a "non-op" race car or show car...and you wont be able to drive your car away from the shop either...it will have to go back on the trailer, its a lot of hassle for a simple part, which is why you will see "race pipes" as a "track only" part, good luck getting around inspections as well if you live n a state that may not have a smog requirement but inspection requirements...and no state in the US allows you to remove at cat unless the vehicle is a non-op race car, show car, or a farm vehicle. Your local laws may not mention it but it is a federal law.
You can not believe me or try to argue what Ive stated, but simply put this is pretty general info ...go google whats out there, talk to guys who are non-turbo/nitrous/supercharged with small C.I. engines and see whats out there, even the honda car tuners these days have done away with running hi-flow cats and have gone to other methods of tuning and finding HP for the street"