52" Mini Z power/cut concerns

[Bayou Boy]

Hello everyone. First post here but I've been viewing for 3 months. Great site!
Demoed a 52"mini Z yesterday. 2006 I think (serial #06031526) w/20hp Kohler. Terrific handling based on limited experience w/bushhog estate, 18/44 fastrack, and Exmark Laser HP. I was ready to purchase this mower w/ only 77 hours for $5000 untill I cut some grass. It seemed to bog down too easily at 2.5". I known its short but the grass didn't really need cutting so I set it bit lower so there would be something to cut. The blades were a bit dull and had never been sharpened/changed. It didn't bog at 3" but the grass couldn't have been more than 4" tall in dry conditions and not thick at all. I have St. Augustine at the house and it gets thick. I left thinking this 'commercial' machine was way under powered and should walk right through most grass. Are my expectations way off or is there something else to consider. I think this is a great deal on this unit and I might get it for less but if it can't cut my grass then its not a great deal. Any feedback would be appreciated.