What kind of exhaust system is on the car? A wideband sensor only works at idle if the exhaust system is air tight. If there are any leaks then the recorded AFR isn't going to be accurate. Open headers are obviously very leaky as are race headers with slip tubes and old headers with warped flanges.

A good wideband will work okay at WOT with open headers but you really need a data logger to capture the information. The LCD screen won't be able to update fast enough to tell you much on a race car. The LCD type of gauges work okay for a street car with a tightly sealed exhaust system and a mild camshaft but they don't work very well with race cams and race exhaust systems.