If you wouldn't mind a comment from a simple mechanic:
In my world I try to make what you term as SOPs into Procedures. However we too have the same dilemmas of non-adherance.
In the main this is also because the procedure doesn't match the task or is too long winded and can be bypassed in some way, and often more legally than illegally. I am constantly trying to get managers to match procedures to tasks - not tasks to procedures.
I also try to cut the need for procedures with the following requirements:
1. Important tasks
2. Rarely done but important tasks
3. Complicated tasks
4. Safety tasks (Business or Health - it's up to you)
Anything else is on a case-by-case basis, but if they dont fit into the above...
K.I.S.S.
Rigga