2.0...ish, But for REAL this time.
You're getting more patient with newbs, I'm impressed! 
OP- the idea that a 3" exhaust connected to a 2.5" turbine outlet is flawed. You're right that 2.5 is smaller. But if you run a 2.5" pipe all the way back, and measure the backpressure at the outlet of the turbine, and then do the same test with a 3" pipe all the way back, the 3" pipe will show less backpressure on the turbine. Less backpressure will mean a higher mass flow rate at a given boost level, thus more power and better spool. Sure going from a 3" to 8" exhaust won't make much difference for the reason you mentioned, but going from 2.5" to 3" is well-proven to be worth it.
OP- the idea that a 3" exhaust connected to a 2.5" turbine outlet is flawed. You're right that 2.5 is smaller. But if you run a 2.5" pipe all the way back, and measure the backpressure at the outlet of the turbine, and then do the same test with a 3" pipe all the way back, the 3" pipe will show less backpressure on the turbine. Less backpressure will mean a higher mass flow rate at a given boost level, thus more power and better spool. Sure going from a 3" to 8" exhaust won't make much difference for the reason you mentioned, but going from 2.5" to 3" is well-proven to be worth it.
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Good, it's good to test things and see what works and what doesn't. I can tell you this has been tested many, many times. Both on miatas, as well as many other platforms. In fact I'm almost certain there's a thread discussing this exact topic (2.5 vs 3" exhaust on a turbo miata) on this site with dyno graphs already, but more data is always welcomed.
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Yeah, we all get to watch. Like I said, if you don't believe or just want to see for yourself, test away. If you really want to be a scientist, datalog the runs, make sure the boost pressure/AFRs are equal between the runs. It's going to lean out if the tune isn't changed when you go from 2.5 to 3" exahust.
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Senior Member
Joined: May 2016
Posts: 584
Total Cats: 95
From: Lents, Oregon
I really got to see this. FYI I am leaving my 2.5 exhaust intentionally just to slow down blowing my engine just a touch longer when I get my track speed kit. Will be going to 3" like a smart person as soon as I build a motor and find a 6 speed.
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Senior Member
Joined: Jun 2015
Posts: 817
Total Cats: 20
From: TAMPA, FL
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