You're cruel, you are!
Exactly what I believe to be the case--I can't test turning for 50ms because the time between the server receive the packets from the start and end of the turn will be greater than 50ms.
No you cannot test using time, so test using angle.
Do a turn using the keys and note down the lowest speed you reach. You have all the time in the world to note down the start and finish angles.
Now repeat using the rudder slider and attempt to end at the same angle (you might need to repeat a few times to get this right). Again, note down the lowest speed you reach. Is it any better?
In so many situations the actual time taken to complete a turn (which of course you have not measured) is less important than the amount of speed you have lost.
You could even be really clever and triangulate your position to reinforcement points, working out your before and after trajectories and knowing the start point of your turn (by noting the time) make a pretty good guess how long the turn took to complete.
Exactly how bored are you?