Waspo is right, that seat and the gun turret are way too many polys, and you should be able to optimize out quite a few on the bottom faces of other things. A simple boxy shape like that with a machine gun should only be around 200 at most.
You cant get it any lower than that of which the hatch is sine i used a 6 sided cylinder with height segment of 1. and for the body i can lower it just a little but not much. Also Waspo an APC is about the same size of a humvee. I can try to get it below 300 but not down to 200 since that will be hard to do.