More impedance is safe for the amp but will result in lower power to the speaker. With just these pics, its hard to say hiw it will sound. It won't hurt to hook it up so try it and see.

Why not just get a powered sub and hook it into the existing speaker and have it on the floor under the pin. That way your not taxing the game amp. I have an 8in. 100 watt sub on my Space Shuttle and it REALLY rumbles and it's adjustable too. Took me about 20 min. to hook up and it's removeable too.

As nice as that idea may sound, personally I avoid mounting subs in the game cab. The constant low frequency vibration is going to tax your hardware quite a bit: may loosen screws, shake out bulbs or even break filaments and anything that moves is going to be stressed. Make sure all your wiring harnesses and connectors are locked down tight if you do.