Hello Jeff,
It's difficult to say what wrong without an log on debug level. It could be that your Midi Fighter Twister is sending range messages instead of relative messages. You can try that very easily by just adding the mode parameter to your mapping.
It would look like this:
{ ch = 1, cc = 0, type = "enc", mode = "range", dataref = "sim/cockpit2/controls/elevator_trim", modifier_up = 0.0125, modifier_down = -0.0125, value_min = -1, value_max = 1, value_wrap = true }
When you commands instead of a dataref no negative values are required at all. You just have to define command_up and command_down. A correct mapping would be:
{ ch = 11, cc = 1, type = "enc", command_up = "sim/instruments/barometer_up", command_down = "sim/instruments/barometer_down" }
Please enable the debug mode in the settings, turn the knob a couple of times up and down and send me the XMidiCtrl_log.txt, please. Then I can see what kind of messages your device is sending. I'm pretty sure that are using the range mode.
Have a nice day,
Marco