good exhaust system comes with LOW BACKPRESSURE , HIGH VELOCITY gas flow !!!

Like I said, it is the rpm which is important.

For example,

1L engine @ 3000rpm produces n m3 of exhaust gas & needs 1" exhaust piping (no mufflers etc) to vent the gas out most quickly.

1L @ 4500rpm -> n+1 m3 & needs 1.2" piping.

1L @ 6000rpm -> n+2 m3 & needs 1.4" piping.

So, piping must suit the volume of gas produced at the relevant rpm.

So, what is the "relevant" rpm?

For manuals, that's whatever rpm you want it to be.

For automatics, it's usually between 2000-4000rpm. Rarely anything out of that range for 99% of the time. So, in order to have the best piping for this 1L auto, you'll need something between 1" to 1.2".

So, why do most people think autos need "backpressure" and restrictive exhausts or additional resonators?

Simply put, because they go listen to the exhaust shop and install piping which is too big, for eg. 1.4" which will be fine for high rev 1L manuals but not for autos which don't high rev very much.

What happens when they go back to the exhaust shop complaining about low rev sluggishness? Will they replace the piping with a smaller one? No, because that confirms they are not very good in exhaust tuning and they can't charge you $$ for the additional work. So, they make up a fairy tale that "autos need backpressure" and recommend restrictive exhausts or additional resonators to make the entire system more restrictive. For eg, adding a restrictive muffler to 1.4" piping will make it flow as if it is 1.2" piping. On top of that, they make $$ selling you mufflers and resonators.

So again, "backpressure" ie restriction is NEVER good. You simply need to choose the piping size that flows best for your engine's capacity and rpm range. If 1.2" piping is best for your auto, then make sure your free flow exhaust and resonators also have internal diameters of 1.2".

So, im driving a MyVi automatic 1.3litres. With stock pipes, exhaust manifold, mid box but with a after market in house made high flow S-Flow.
So if i were to change my manifold to a 4-1 header (The only currently offered configuration to date), maintained the stock pipe and mid box but changed to sports exhaust / free flow exhaust at the rear, confirm i won't lose any torque / power at lower / higher rpms?

Please advise.

From my understanding 4-1 extractors will boost ur mid to high range rpm but u will suffer from low to mid range. 4-2-1 improves low to mid but lags torque on the mid to high range compared to 4-1.