Some say it makes a difference. Some say no difference. K&N doesn't claim improvements with the drop-in filters, though most seem to see very modest gains. K&N's main appeal is that the filter is cleanable and reusable for one million miles (20 cleanings every 50,000 miles) and that there will be 19 less filters in the landfill after that amount of time. Your filter part number for your 2004 ST is 33-2106-1 and will run you about $43 on-line. If you are looking for more performance there are the air intake kits offered by several manufacturers that replace the stock restrictive airbox. K&N's FIPK part number 57-2529-1 is the kit for your ST and they claim almost 7 HP at 5200 rpm. This kit will set you back $229. It also uses a cleanable filter (conical). You will also need a filter cleaning kit to clean the filter, part number 99-5000, $10.