Well, Gex-X is supposed to have a massive submersible pump, but I've never seen if for sale. You can see the specs at Pacific Coast Import's website. However, I'd be worried about heat.
Another good submersible pump for pressure is the Sen 9000, but it is noisy.
Bottom line is that you probably won't find a submersible pump to match the best of the external ones (like the Iwaki).