ADS7924 sampling rate

The website advertises 100ksps, but cleverly does not say "Per Channel".  This is also in its fastest configuration.  In fact, you can achieve 100ksps with this device only in "Auto-Single Mode", where the ADC automatically converts on a single channel according to the acquisition time set by the registers.  By selecting this mode, and programming a 6us acquisition time (the minimum), one conversion takes 10us, or 100kSPS.

However, this throughput rate drops significantly if you try to read out the results via the I2C interface, as you calculated.  Also, simply by monitoring all four channels, you are down to 25kSPS, as you noticed.

The ADS7924 is designed to be used as a stand-alone monitoring device.  After loading the registers with your desired configurations, this ADC automatically wakes up, takes measurements, compares them to programmed limits, then goes back to sleep.  If a sample is found that is outside of your programmed limits,  the INT pin is toggled, which signals your host controller to take some action.  Therefore, by using the 100kSPS configuration, you could react to an out-of-spec condition within 10us of the event, all without continuously using the resources of the host controller.

This device can do 10kSPS per channel, but you would not be able to read back the data for each conversion.  After programming the device, your controller would simply be monitoring for the INT signal.  When that happens, the ADC stops converting automatically, and you can read back the conversion values of the last cycle.