They "buffed" Pharah, sure.
But they also buffed Soldier:76 and Widowmaker. Yesterday Taimou tried out Soldier76 on his stream, and he had over 35,000 damage on King's Row with him! So now for Pharah there's not just McCree to worry about, there's also Soldier:76 returning into action (and looking very strong). And of course Widowmaker, Ana, and even Mei. This just means playing Pharah will be extremely challenging vs. those who have good aim.
With regards to McCree and aim, I recommend going through a long list of things to practice, first at practice range, then 1v6 Ana bots on hard setting, 200% damage, 25% respawn time, headshots only.
First of all make sure your Windows mouse settings is 6/11, acceleration disabled. Then the DPI should be something like 800, and sensitivity from 4 to 6 or so (plus minus), polling rate 500 or 1000. You should have a large mousepad, for example Zowie G-SR. Since McCree is hitscan, you want to have precise aiming, and you won't get that with high DPI and high sensitivity.
Personally I run it at 800 DPI/4.66 sens, and it works quite well for me.
Now, as for the practice itself, there's quite many YouTube videos on how to improve aim in general, and more specifically for McCree. You just have to put in at least a couple hours every day with practice only (not playing the game itself) then it'll improve. You'll find that when you play the game, be it quick play or ranked, you'll do much better than before.