Relationship between clock speed and I2C bus speed
Except for the overhead it takes for the Start and Stop conditions, acknowledge bits, and device address plus R/W bit, they are the same.  

So the more data that is sent per message, the closer the data rate will be to the clock speed.  For very short messages (one address byte and one data byte), the data rate will be a little less than 50% of the clock rate.  For very long messages, it will be more like 90%.