The pre-filter is a piece of filter foam that lays over the top of the filter element. It is the exact size of the oiled gauze part of the K&N. It comes with the filter. If you don't have one, the PO probably tossed it away.
I talked to a K&N engineer years back concerning air flow with & w/o the pre-filter. He said the air flow is about the same as stock using the K&N with the pre-filter & air flow is increased w/o the pre.
Conclusion- with pre-filter no performance gain, no rejetting basically just a long life cleanable filter. Without pre-filter you may have to re-jet especially if you have other mods like free-er flowing exhaust.