ActiveSync port ipad

There is some specific information on the components involved and what is being reported by this error in this technet article. Given that you are certain your own firewalls are not enforcing the session timeouts it is pretty clear that the most likely culprit is the Cellular provider.

The primary effect that this will have is poorer battery life for users with relatively low mail volumes, for high volume users the mail downloads dominate the power consumption so power users tend not to notice these problems as much as those who only get infrequent mail. The article indicates that it will lead to missed push notifications - what will happen is that notifications that happen during the timeout period will not result in a push and mail will only arrive after the client realizes that the heartbeat has been lost and reconnects. At least some of your users should be seeing some mails being delayed. The delays should never exceed 30 minutes and correctly functioning device should then reduce the sleep interval until they find a session duration that is sustainable.

Also note that the this is an average number - many (even most) of your users may be perfectly fine and the reduced average may be caused by an issue affecting a minority of users.

ActiveSync and IMAP are quite similar. Both ActiveSync and IMAP IDLE maintain a dormant TCP session for inbound push notifications. IMAP IDLE handles 'push' email as well FYI.

Both are quite conservative with bandwidth usage. Bandwidth consumption in practice will be more determined by configuration parameters than protocol choice. Setting things like 'only download first 50k of email' or 'only download attachments in X state' will make large differences in consumption.