While looking for an AC to fly across Australia I came across this fix for the AI Cherokee: piper_fix.zip from avsim.com:
"A fix for the default AI Piper Cherokee. After watching some AI traffic I noticed that the default Piper tends to drop like a stone on landing - so I have tweaked the supplied flight dynamics file to correct that. I have tested it and the default Piper will now land alot more realistically when used as AI traffic."
Way better landing 8-)