Based on my experience, if you can indeed find the studs behind the plywood and drywall - you will be good to go! Sometimes there isn't enough gripping "depth" in plywood itself. 2 1/2" - 3" lag bolts will do a great job in mounting the bracket to the studs. You might want to make sure the back of the cabinet is completely flush with the drywall behind it. Simply push on the back of the cabinet and make sure it doesn't give at all. If it does and you mount to the plywood only (at any point), you could cause some problems when the flat screen is hung.
I often use low profile mounts for installs like yours. They allow a tight, close install in the cabinet and still provide some tilting ability for maximum picture quality.