Read up some some stuff about DPI and mouse settings and my McCree accuracy has gone from 35% to 45-55% as a result. Played a shitload of McCree over the weekend, much to the chagrin of my teammates. Comp is 5x DPS? Sounds like time for McCree. No healer? McCree. Mercy and Ana on tap and no tanks? It's high noon.
After playing a good bit in the tail end of S2 in the low 2000's, I've decided that most bad players choose DPS and that outside of basic comp issues, bad DPS is the cause for most losses. If you take for granted that someone will generally be guilted into playing at least one tank or healer, I think getting really good at a single DPS might be a way to carry yourself out of low ELO. If people even complain for a heartbeat, just mute them.
I haven't read a lot of the earlier posts in this thread, but a real good approach to training aim (after you figure out your mouse settings) is to set up a custom skirmish game with headshot only, 200x damage and fill both teams with Ana as AI (Can't HS). You'll get infinitely more work in practicing aim/headshots in with this approach than playing in comp. Doesn't help a ton with with tracking Pharah/Z axis or getting work in on rolls/melees/special moves, but it's been super helpful for me.