Just how much does one's shooting degenerate in a real, stressful situation?...
Speaking from real world experience...a LOT!
That's why it's pretty important to get a lot of practice in with regards to both shooting and just plain gun handling.
Finish an incident and you'll be amazed at how tough some things can be such as simply re-holstering.
One of the best training tools I've found is to tape a cheap laser to the gun and practice dry-fire (obeying all safety rules of course). You'll be astounded at how that laser dances!