I ride with a SON dynohub, 6V 3W. I do notice when I have the additional 3W load when my head light is on. I'd love to have a 15W dynamo and headlamp but the load would be so great I'd probably end up going backwards.
I don't recall seeing a 2.4 watt dynamo, just 3W dynamos then 2.4W and 3.0W headlights and 0.6W taillights. Are you sure the dynamo is rated to 2.4W and not the head lamp?