I have the ZD331 72" and cut about 5 acres, mostly bahia grass (fast growing and tough). Prior to this machine I had a Exmark 54" 25HP gas. The Exmark, is a nice machine for a "normal" yard, it just didn't cut it (excuse the pun) on this property.
The ZD331 has more than ample HP and is just an all around heavier duty machine. This machine with the 31HP, will wreck the OEM blades when cutting tough grasses. When used with Gator blades, this thing can't be stopped.