I have the manual 6-speed so I'm not too certain about the autos. My son has an SRT-8 auto and the diablo tune. It improved the shifting quite nicely. The trans should be the same one used in the SRT-8s and they have 425 HP so I believe the 5.7s increase to over 400 HP should be fine for this transmission. I have read articles on 300s and Chargers with over 600HP and no trans problems yet and all are autos.