Kevin,
The lead in the gas leaves tiny deposits of lead throughout your system. The lead specks will coat the O2 sensor and cause it to react slower and slower to air/fuel ratio changes. If you have an A/F meter you'll see as time goes by the meter will fluctuate up and down at a slower and slower rate. Eventually you'll need to replace your O2 sensor with new ones and replace your spark plugs.
If you've run the leaded gas already then you'll see the little specks of lead on your plugs when you change them.
If you must use unleaded then drain the tank, pour in the leaded, run your race, and then drain the tank and pour in your unleaded. Then if you're real picky about the O2 being correct because of how your car is tuned you can change the O2 sensors after every 5 to 10 races.