Would the +100W TDP (450W) affect mining efficiency or can the power limit probably just be tuned down more than now without a significant loss?
Are you asking about the RTX 3090 graphics card?
The optimal solution for mining this video card is 114-120 megahash with a consumption of about 300 watts.
You will not be very successful if you add the power limit.
This will only lead to additional overheating of the video card.