I’ve been told that “Air is lazy” and will more readily flow around an object instead of through it. If your existing radiator isn’t fully shrouded and sealed around the full perimeter, air is escaping around it instead of through it. This will greatly diminish the effectiveness of your radiator both while sitting and moving at speed. The only reason to worry about increasing the fan airflow rate is when your sitting still or barely moving. If you were having issues above 30mph or so, try making sure there aren’t any ways for airflow to escape around the radiator.