Mark,
It is important to understand how Total Handle Time is being calculated in Statserver. From a stat perspective, there is no differentiation between reading-time and responding-time. The whole time would be represented in the time the Email is active with the Agent. This would be analogous to "Talk Time" with a phone call; I am listening and talking. Once the email is sent, it is possible that there is some internal follow-up work that the agent must take care of before they are ready to receive the next email. This would equate to ACW. Both of these added together and then divided by the total number of emails handled would give you your AHT.
Can you confirm for us that this is how AHT is being gathered out of Statserver?