Here's a little trick I use fairly frequently to figure out undocumented information. Download RegMon from Sysinternals.com and open up the Exchange Manager. Navigate to the POP3 Connector config screen. Then run RegMon. Next, change the retrieval interval and Apply the change. Stop RegMon's logging and look at all the registry events that are related to Exchange. Comb through and see if you can find a registry entry that changed when you changed the interval. That will be your reg entry.
Next, go to that registry key and see if you can change it to something lower than 15 minutes. The actual key may use seconds, not minutes, so you may have to calculate that out. If the reg key has a hardcoded lower limit, you are out of luck, but if the limit was imposed by the GUI, you may be able to bring it down to 5 minutes.
And be careful...this is the registry.
ShackDaddy