Hi Everyone

Before I continue, I would like to state that I am not familiar with the science behind exhaust setups, so please be kind with my lack of knowledge!

Also important to note that I live in Johannesburg, South Africa where the altitude is 5700ft above sea level so NA cars really do suffer here.

On to the question....
I had a 2012 C63 with 4 into 1 headers, into a centre box with an x pipe in the box, straight to the rear mufflers (no resonators), and a tune.
With this setup, the car made 302wkw (405whp), and I can't remember the torque.
The car managed a best 100km/h to 200km/h time of 8.9 sec in this setup.

I wasn't happy with the sound, so I got them to remove the centre box and just put an x pipe in its place.
So the setup was 4 into 1 headers, into an x pipe, straight to rear mufflers, still with the same tune.

I noticed that the 100km/h to 200km/h time dropped to roughly 10sec.
I figured that the tune may have had to be refined, but never got around to doing it and ended up selling the car.

My brother in law now has a 2013 C63, and we just had the exhaust and tune done.
His setup is the same as mine was before I sold it.
So 4 into 1 headers, into an x pipe, straight to rear mufflers and a tune.

I did some pulls last night and managed similar times of 10sec from 100km/h to 200km/h.

So my question is:
Are we doing something wrong with the setup? Is it more effective to have a centre box over just going straight to the stock rear mufflers?
I cannot confirm the exhaust diameter as my tuner arranged everything. But looks like 3 inch headers and the rest of the system being 2.5 inch.

My brother in law isn't very interested in power, more about the sound.
It just got me wondering if we're doing something wrong with the setup.

Thanks!
Kimeran