The standard filling method for the gearboxes is to fill it till it runs out. If you remove the filler and it doesn't come out.. not enough fluid.
From the sounds of it you may have clutch issues.. possibly glazed over by the old fluid in there.
Another possibility is its registered a major code to the computer and in limp home mode. Get the error codes from the OBD to find out whats really going on. I doubt it would uncode itself tho... if it does.. that may cause issues.
Yes... i'm a total car nut! Why else would I upgrade my mower to rev to twice its standard rpm!