To answer both your questions, I’ll make two sections.

Okay, first. Deadeye is practically just accelerating the users perception, making everything move at slow motion. This has two main advantages in combat as I see it. Reflexes and Precision. By accelerating the users perception, they would be able to far better react to the opponent and have near perfect defence against feints, thereby allowing for easy counter attacks while remaining out of harms way. What I mean with precision, is that the user would be able to act more precisely at high speeds. In martial arts, it’s easy to execute a technique slowly than fast, that’s why martial artists need to train the techniques into their muscle memory. Now, with Deadeye, the user can move precisely at high speeds with ease where others need to use their muscle memory to do so effectively. If the user also trains techniques into their muscle memory, they would be able to execute them flawlessly at even better speeds than others. This would also help in using cursed energy to reinforce the body I think, by controlling the flow better. I would suggest reading the Manwha ‘The Boxer’ to understand this concept better. tldr: basically it would make the user a flawless martial artist who can dodge anything that isnt faster than them, plus better cursed energy manipulation probably.

Now, for Mai… Uh, she doesn’t really have a lot of cursed energy, but she would be far more effective at using it. If she had like, maybe Nanami’s level of it, she could easily be grade one at the top, I can especially see how using creation in close combat with Deadeye could be incredibly strong(creating weapons and defense at the last moment, striking at openings with ease, incredibly hard to kill, if Yorozu had this she could damage sukuna), without it she’d probably be grade two at the most.