If you dont have enough back pressure the car won't run.
it needs enough back pressure so as not to work as an exhaust brake, what a truck uses, and not to little as to let all the petrol flow out the exhaust port.
thats the use of a tuned pipe to allow adjustment of the back pressure so that enough fuel/air will stay in the engine.
you would need to run testing too see if there is more or less back pressure.
It is very hard to tell really, but normally, with that pipe you would think there is a higher back pressure, but i takes longer for the exhaust waves to reach back to the motor, so its quite hard to explain a 2-stroke exhaust.
The re-bound of the exhaust waves tho is probably from the T-intersection near the back of the car.
Its very technical.