My 340Da when slowing down to an idle from plane will throw an alarm and go into limp mode. Oil pressure is good when this happens. If you slow down below 1k rpm for a few seconds it will be fine. I'm thinking oil pressure switch not oil sender switch is the problem.
Depends who you talk to but yes they will help with lower rpm for planning speeds and increase speed at a lower rpm plus help give you more thrust when docking. How much of a difference I doubt if it's huge but over time it will be a savings.