By doing so however, the machine will always run at whatever processor speed you select.

If you create a custom power scheme and set the battery processor performance to highest, it will always run at that speed.

From what I understand there is no way (via software) to force full speed at startup then drop into lower processor speed once booted and logged in. The only way you could do this it to boot connected to AC and unplug once booted, thus dropping into reduced processor speed mode.