I'm more inclined to question an issue in the trans. If it's losing that much rpm that you have to feed a lot of throttle in one direction more than the other, it may be the linkage inside the trans is applying a band or clutch pack at the wrong time. How is the trans shifted? Has the linkage been abused, beat on?

Check out Denny's post / questions. If that doesn't help, consider having the trans linkage investigated.