A narrorband sensor will still say its rich whilst the top of your pistons are rattling around in the crankcase after meltdown, thats why They just dont have the capability to read mixture at WOT as they are designed for mixtures concentrated around 14.7:1 and are not accurate at all with very rich mixtures.

The one Jesse has linked to is a decent one, and ive tested it against my own £6K+ machine and its reasonably accurate