I'm not sure what you mean by "couldn't achieve proper functionality and performance". For a turbo, bigger is always better. Backpressure is the devil for turbos. A dump tube off the turbine housing would be the perfect turbo exhaust, but it would be insanely loud. The flow difference between 2.5 and 3 inches is pretty big. It's not straight math.
It may not be as noticeable as making X power vs Y power. If its running more efficiently, you may make the same power on less boost, leading to a cooler intake charge and a less stressed motor.
Most companies use 2.5 inch exhausts because they can sell them to every 370. A 3" exhaust on a NA 370 would most likely be way too big. As few turbo 370s as there are out there, its hard for a business to justify making an exhaust that so few people can use. There's only GTM and FI as far as I know.
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2011 MB Touring-Sport-6sp-Nav/GTM TT/FI TT TDX/JTran/Kosmic/Eibach/Hotchkis/SPC/CSF/RPS/SoThatsWhereAllMyMoneyWent
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