Steep works really well for mountain bikes and mountain biking, because they let you climb better. They also let you get the saddle out of the way for the downhill parts. These are all things that benefit us. However, I think that if you do a lot of non-vertical riding there will be a limit to how steep you (could be different for everyone) should go, because on flat ground you can still end up with your weight balance and body angles not being as comfortable or useful.
The faster you want to go on flat ground, then I think the slacker you want your STA to be so you can be more aero and have less weight on your hands and upper body muscles by stretching out. Riding this way you can cover more ground at a better pace because you are more comfortable. But then again, if you are ride your road mostly up mountains, a steeper STA might help. But unless you have a dropper post on your road/gravel bike, you may not like the position going down.
I can get a lot of power from behind the BB unlike some people here feel, but I think that is only the case if I have the right reach to the bars. Peak power comes from using the whole body and if the reach is wrong (likely wrong if it is too long) then you won't feel as powerful.