Pretty sure you did perfectly at wording it, but just to make sure:

"If you stand in the middle of the street in broad day light, zombies will be able to see you and the game's default detection system will apply."

Default zombo sight is normal but I play on poor bcuz I dont like when they spot me a screen away
Will this change their sight level to normal or will they stay at poor that I've set in sandbox?

Since you specifically said "default detection system" and not "level"
I'm hope that means they stay at the sight level I've set in sandbox

Damn, looks like it does overwrite sight settings :/
Don't want zombies spotting me from 2 screens away
And fog actually feels worse then my preferred sight setting w/o the mod because of it .-.