If you users use certain ip ranges you can set them as trusted ip addresse, so any limits won't apply to them.
Unfortunatly I am sure they don't, so you will need to setup up a second port or hostname with different download speed settings in place that is for use by premium users only.
In order to enable throttling, you need to set serverwide configuration to none-zero, then set the limit for the vhost that don't need to be throttled to a large number.