Data packet format when USB initialized as COM port Does anyone know how the data in data packet looks like when the USB is initialized as COM port?

I have a Arduino Mega 2560. I have initialized COM port with a baud rate of 9600. I know that there are four types of USB packets as 


Token Packet (Commonly used for device identification and transaction initiation.)
Data Packet (Contains the data that needs to be transferred from host to client or vice-versa.)
Acknowledgement Packet (Acknowledgement for error-free data reception.)
Special Packets (Hub to hub communication and speed differential.)


The payload in data packet is formatted as 

DATA1 >> PAYLOAD >> CRC16 ; DATA2 >> >> PAYLOAD >> CRC16 ; etc.

Till this point I am clear about whats happening. 

Now all I want to know is 


How the data is structured in the PAYLOAD section, when USB is initialized as COM port?
How the baud rate comes into picture i.e how a particular baudrate is achieved in this type of communication?
How is communication in terms of packet sequence? Does the sequence of packets change? (Like token>>data>>ack>>data>>ack>>...)


I really need to know this urgently as I am working on program that has to connect with 3 MEga2560 boards through a USB hub. I want to fix my design constraints depending on what is the maximum baud rate that I can achieve for all boards.
Once the USB connection is initialized as a COM port, it no longer utilizes traditional USB packets (only internally). It acts as a regular COM port. This layer of abstraction is created by 8u2 chip on Arduino side and by the driver on the PC side.

If you need to use the USB without the COM port abstraction, you have to probably configure the 8u2 chip to behave this way. I think it should be possible to configure to act as some HID device for example. You would also need to take care about the PC side and you might have to write your own driver.

For a guide on hacking and configuring the 8u2 chip on Mega, see following link:

http://arduino.cc/en/Hacking/DFUProgramming8U2