470 each max...
Yup, that should be about as good as it gets with air cooling.
They are essentially 7870's, though they tend to have slightly better memory bandwidth and overall performance. Might be due to more refined bios rather than some difference in hardware, I haven't found any clear explanation for the documented (minor) differences in performance.
In terms of hashing, they should perform very comparable to an equally clocked 7870.