No stats nor code for the ATV exist, so it is impossible to know how the APC behaved. The only sign that the ATV existed at one point is that it's HUD image can be found in the game's files. Why it was cut is unknown. On the Steam release, they uploaded photos from the Beta of Serious Sam 2. One of the photos shows three ATVs being used by Orc Gunners and one Orc Commander (shown below).
The ATV was seen on the end mission Unreal Wasteland.