I am aware of all this, and you are right. Even getting real 16 bit is tricky and forget about doing it really fast.
As to mistaking sample frequency and output data rate, while I did not read the data-sheets, if you look at the first page of the one for the ADS1675, you find "4Msps", which should be 4 million measurements per second. You will also see that it has actually up to 32Mbps I/O data rate for its digital interface. If I read this right, they already do 8x oversampling (hence you can _read_ in fact only 500k measurements/second) and are at roughly 16 bit resolution due to noise. And no, I do not know how they manage to get this number of measurements from the signal the sigma-delta modulator gives them at these "low" clock rates, but some advanced trickery is clearly involved.
Now, I might have misunderstood something here completely, if so, please enlighten me.