That doesn't make sense at all. Why fit 3" exhaust if your going to fit more mufflers to quieten it? It's actually mufflers that cause a restriction

Um well first where talking about a NA Ford 4.0 I6 not some high performance turbo engine.

I'm not just some kid with no knowledge giving my opinion, i'm talking from years of research training and experience i've only been a fully qualified mechanic for 7yrs but have a lot of knowledge from working with guys that have 30+ years in the trade. I don't care what your dyno read out says, dyno's do not measure real world performance! They measure the amount of maximum power your engine makes at the rear wheels. So because a 3" inch system might make a little more power at max revs does not mean anything, do you constantly drive at max revs, is this a race or drag car or just your daily drive? If it is for a daily drive it is overkill and not to mention will sound terrible

Where did you hear that your 2.5" system is too restrictive? If anything it would be your mufflers causing a restriction not pipe diameter, a good section of straight pipe will flow around 115 CFM per inch of diameter.

Exhaust gas is hot and it's best to keep it hot throughout the exhaust system. Why? Cold air is dense air, and dense air is heavy air. An extremely large exhaust pipe will cause a slow exhaust flow because the gas cools and becomes heavier

Mass of air that the engine breathes in + mass of fuel = mass of exhaust gases. A 2.5" diameter is well suited to a 3984 cc engine that revs to only 6000rpm. Turbo or supercharged engine is total different story

The problem is exhaust shops are just fitters most do not know much at all and they will tell you fitting 3" system will improve performance.

Without being forced induction a 3" system is just over kill. I've had major head work done to my 95DT head by Gilbert's P & A Cylinder Head Services, these guys really know the EA-AU 3.9/4.0. I've tried like every off shelf system made for EL 4.0, it's taken me years to make a system that flows very well but also sounds good. I running 4480 pacemaker headers 2.5" redback high flow cat, 2.5" mandrel bent pipe into hotdog just before diff then 2.5" mandrel bent tail pipe with a mercury 2.5" high flow muffler cut in. This system sounds nothing like an off shelf sports system, it has no drone at all it is actually pretty quiet highway driving. Has a real nice lumpy growl not that horrid droning bark sound most falcons with sports exhaust have, sounds really good echoing off buildings even turns heads

One of the exhaust manufacturers, maybe redcat, make a 3" tail pipe section for over the diff.
Buy one of them then the exhaust shop only needs to knock up the centre section, very easy for them.

If you are refering to my post about getting a bit more torque with the header lengthening, it will be getting tuned after i get it done to suit.

Let me whisper a little something in your ear.. its not 'just' the exhaust, its fairer to say that a engine that cant adjust its fuel delivery needs back pressure to perform.