A friend had a K&N filter to replace the stock filter. This was using the stock hardware. All I had to do was remove old filter and replace with new K&N filter.
It just so happened the local H-D dealership was running a special on dyno runs so I decided to run the stock and then switch with the K&N and see what difference I might see in filters. No other changes, just run stock, then swap in the K&N.
On my '02 1200 XL.
HP I gained .5 with the K&N.
Torque I lost .8.
The small differences didn't justify the cost of the K&N.
He wanted $65.
Later I did do a complete Stage 1, using both the SE A/C and SE mufflers and also doing a change with the jets on the Sportster.
Then it was noticable and worth it.