I have the 800 also and run it with an pressure rated Iwaki 55. That pump is more than enough for that skimmer, in fact I have to cut mine back.
Something clearly looks wrong in your picture. It looks like you are very underpowered. The water/foam line looks like it is 2"-3" up into the body. If I was to run my Iwaki 55 full throttle, then close my gate valve until the water/foam line was as high as yours I would have a flood in a matter of minutes.
If the pictures are with the Mag 12 and you aren't seeing a difference with the Iwaki, then something is wrong. My gate valve is set so there is NO water in the skimmer body. The only thing I see is foam. After I feed the fish there is no water or foam in the body even though the pump is still going full blast. So for you to have that much water in the body tells me you're either not pumping enough water through or getting enough air in the mix or both, meaning you may not be getting enough air in the mix because you are not pumping enough water through it.
Here's a couple questions:
-Are you cutting the pump back?
-have you cleaned out the air intakes?
-have you cleaned the Iwaki with vinegar?
-Is the Iwaki 55 the pressure rated model?
Finally, again, what bioload do you have in the tank?
I've read good stuff about the air inlet mod, but I think you need to solve some other problem first.
Greg, I have a vent on my return that I ended up blocking off. The only difference I noticed with having the vent closed is the water moved through a bit faster with the siphon, but I just closed the gate valve a bit. Made it a lot quieter though.