This is a great effort dude, but a few things bother me. I kinda feeli in my opinion that the background is distracting the viewer from the model. As the background is a flat image, it does not have any depth. A few things I would try if you are set on using this background are:
in real life, the furher back you go into the distance, the lower the contrast gets due to stuff floating about in the air. If you have Photoshop I would try to mimic this effect.
some depth of field would really help. Obviously you cant use Blender for this, but again, using PS to blur your background in relation to its depth would make it feel more realistic and also help to showcase the actual model
The lighting on your model makes it hard to see and you miss all the details that you went to all that effort to make.
I think the background makes the scale of the Mech look off. If you look at the car nearest the camera and try to imagine the size of a person and then think of that person walking the few steps to your mech, then the mech would be fairly small and I cant see the person bieng able to climb into that cockpit and fit inside it.
Perosnally, if it was me and I was trying to show off the model to impress a company like you said, then I would model my own fairly simple scene and ditch the background. Although your compositing is fairly impressive as the mech and shadow do fit into it well, it still feels like you have chucked the background in at the last minute. If you are going to use the bacground then I would cut a lot of it out and do some of the things I have suggested to make this image about the Mech.
Lastly, remember what I was saying about backstorys and giving purpose to your image. To me , this looks like a sci-fi futurisic mech in a 2016 world due to the age of the cars.
I realise im being quite critical, and this is only my personal opinion remember, but I know how much you want this model to impress, so im just trying to help as much as I can.