I am going to build a high performance PC with very low power consumption.
The idea is to have a system that gives best gaming performance per watt.

For that purpose I chose the upcomming Ryzen Zen 3 4800. I will be disabling SMT + CPU boost so that CPU takes only around 30 watts then I will add a 32GB micron memory kit and overclock them well.

The motherboard will be an ASUS one with epu chip for maximum power saving.

The PSU will be a special 300 watts DC that can accpet wide voltage input range so the system should be powered with AC to DC adapter or through DC batteries (cars, trucks, solar systems… whatever).

The only leftover problem is the GPU.
I believe the RTX 3080 TI will give the best performance per watt if I used power limits, but that is not enough. The 50% power limit will cut down power usage from 350 watss to 150 watts and I want it to work at even less like 100, 90, 80… 60 watts whatever will make performance per watt higher.

With that being said I need a bios mod that will enable custom power limits from 50 watts to 400 watts. Is that process hard, complicated, unsafe…?. Could anyone help me or do that for me?.

I have never moded a GPU bios, I had very little experience with motherboards bios moding.

@Islam_Ghunym - What you want is not possible with Nvidia, due to you cannot modify modern Nvidia BIOS, and older Nvidia cards that you can mod BIOS with are HOT/Power Hungry compared to moderns cards.
So your best bet is to go AMD card, or choose whatever modern Nvidia has lowest TDP/Wattage specs that fit your needs.

Lets hope AMD will have something good then, but I have huge doubts that it will ever have better than Nvidia

Nvidia has DLSS which something that AMD does not own and DLSS is great.

more cuda cores or more stream processors will always mean better performance per watt, right?

No, to your last question. Only direct comparison of similar spec/model series would answer that kind of question, it also may vary from model to model in any given series too due to card design, power delivery/optimizations etc.