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An exceedingly slow part. Ten Pi units together costing $350 would be absolutely dusted by a single $350 full-scale GPU.

A $350 video card would have upwards of 2000 shaders and would run at a much higher clock-speed. It's simply orders of magnitude faster at a price that's only one order of magnitude higher.

Not any more easily than before. Previously you had the full power of the OpenGL ES API to program the GPU with, but if you wanted to do anything that didn't fit into the API you were SOL. Now you're in exactly the same situation, because the "high-level interface" to the GPU firmware is just OpenGL ES with the context set-up and buffer management replaced by something nastier.

With such an open stack, could the Pi be used as part of a GPU password cracking cluster?

From what hermanhermitage said above, I'd guess no. You've now got full access to the OpenGL API, but you don't have full access to the hardware running the binary blob which is what is running the OpenGL code.