Two problems exist with the simple narrow band unheated sensors like the K&N uses.
Major Problem 1 is the standard narrow band oxygen sensor is designed for precision at 14.7:1 . There is a huge slope on either side of the sensor output once you move away from 14.7:1.
Problem 2 is temperature. The sensor output is temperature dependent and an unheated sensor like the K&N is dependent on the exhaust gas to keep it at the proper temp. It's a minor issue but still something to consider.
A good heated wideband oxygen sensor that will read accurately over the range racers are interested in costs upwards of $250, and that's without anything to read the output. I have one of the K&N type units sitting on the shelf. It's a fun toy, but nothing more. Haltech makes a reasonably affordable unit with a heated sensor, but you'll need a fairly stout battery to power the heating element.
http://www.halmeter.com/Halmeter.imgs/Halmeter_AF30_Brochure.pdf
This unit is still subject to the slope issues inherent in narrow band sensors but the display does a better job of providing feedback.
Don't forget that lead and excessively rich mixtures will destroy the O2 sensor in short order.