NefMoto

What is the maximum allowable exhaust gas backpressure? Is 5 psi acceptable at high load?

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.

Yes, my car is turbocharged. I understand what you mean, but I want to find out if the performance of the exhaust is sufficient.
I installed a pressure sensor instead of the front O2 sensor, that means, after the turbocharger and before the cat. I don't even have a clue how I can install it before the turbocharger...

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.

Thank you. There is not so much information about backpressure on the Internet.

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 use SAI hole to measure backpressure

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

If you have 0.3 pre-cat on full load on a turbocharged car you will gain nothing by doing anything to your exhaust. Nothing measurable anyway compared to turning the boost up a little, it will be within margin of error.
On N/A there will be a much more significant difference.