The feature you are looking for is called "Rate Limiting". I think this has been introduced with Webwasher 6.8.7.
Do you have anymore information regarding rate-limiting on 6.875 or version 7.0? I have checked with support and they do not think this feature is in the software.
Rate Limiting or Bandwidth Management would be the next strong argument comparing McAfee WebGateway and Bluecoat Proxies.
Rate Limiting based on Req/s, KByte/s and concurrent connections is part of 6.8.7 (on Appliance).
In 7 you can only use concurrent connections for Rate Limiting at the moment. I am pretty sure more will come in the near future :-)