The very fact that Mac is hiding his aphasia and delegating his reports, case files, and court appearances says he knows damn well it's affecting his job performance. His is not performing to his former standard, and he knows it, and what's more, if this were one of his subordinates hiding a decline in cognitive function from him for the sake of their pride, he'd be soapboxing so hard you could hold a derby and delivering fiery speeches about the integrity of the lab being more important than their fear and wounded pride.
Mac knows his aphasia could compromise a case, which is why he's "delegating". If he were doing it because he'd finally learned to trust his team, it would be laudable and fabulous and the mark of a leader, but as it stands, it's self-serving and grossly hypocritical. He's been a raging douchelord to anyone who's expressed concern for him or told him something about his injury that he doesn't want to hear, including a doctor friend who went out of his way to drop everything and meet him for coffee.
His behavior is understandable from a purely human perspective; people do pissy,terrible things when frightened or ashamed. Understandable or not, it's also indefensible, especially from someone who trots out the Integrity canard at every opportunity, and it's rather telling that his precious integrity was the first card he played when Christine questioned him. On some level, Mac knows that he's wrong, but right now, he simply can't or won't admit it. Just like the myriad crooks of whom he has been so contemptuous. Like Flack with his murder of Cade, he's lost the moral high ground, and he knows it, hence his minor breakdown on the steps.
As Mac's second in command, Jo is as responsible for the lab as Mac. No, she doesn't have definitive proof that something is wrong, and no, she should not have approached Christine before anyone else, but if and when this blows up in everyone's face, she's going to catch a lot of the blowback, and one of the first questions people will ask is, "Were you aware of Detective Taylor's compromised abilities, and if so, why didn't you alert anyone?" Jo's ass will be right beside Mac's in the sling, and unlike Mac, she has neither long standing in the department or a sterling record, thanks to her whistle-blowing imbroglio with the FBI.
Don't even ask how hard I squeed at Flack in an apron, cooking eggs Benedict for himself and his dog.