The less the better.
But the question is not really well formed.

Is your car turbocharged?
Then you should measure the pressure before the turbine. In case of a turbocharged car the 5 psi is a drop in the ocean compared to the backpressure before the turbine, so you can ignore it.

In case of N/A of course the less the better.

If it's 0.3 on full load after the turbo it's irrelevant.
Anything below 0.5 bar probably is not going to be very noticeable.

You most likely have 1 bar+ before the turbo on full load.

depending on your turbo size and boost levels you're running, you might find out that you're running near double the amount of boost in terms of backpressure... backpressure is a very obscure subject because it's not easy to install sensors and measure it, and most people have no clue wtf it means or how it works even. Drill/tap your hotside or your manifold and then you'll know for sure. you'll also need to figure out a way to cool off the gasses before you can get them into a sensor or a gauge of any kind.

I had no idea either, which is why I asked. But they just buy a full Akrapovic. Maybe I'm too greedy or poor.