This is embarassing but we've smoked the tranny three times in a car before we finally figured out that the cooler was plugged. It behaved like you're describing. The motor sounded great and pulled clean; it just took forever to reach shift points and ran about a second slower than it should have. On the jackstands prior to running it would shift fine. A few seconds after launch it would fill the car with burnt clutch smell. However the fluid always smelled and looked fine.

Just a thought.