I used a 425 AWS with a 60" deck for 5 years. The only power issue that I remember, was that it had to be at about full throttle when the mower deck was engaged.
I do not remember ever not having enough power when mowing.
It handles the 60" just fine. I jist mowed extremely long grass behind the shed yesterday was very impressed. Had to run wide open but that's not a big deal.