Any experiences with air fuel ratio meters? Escpecially those that have oxygen sensors that can be installed in the end of the tail pipes.
At Manchester we used an Innovate Motorsports LM2 air fuel ratio meter to measure and set the AFR for each of the tests. The only difference from your description is that the Lambda sensor was fitted into the downpipe not at the "end of the tailpipes". In my opinion this worked very well. It also had the advantage that in a road car, you could use it to monitor and log the AFR as you were driving along allowing a greater range of possible running conditions. The disadvantage is that it needed an adapter to be welded into the exhaust pipe.
I have no direct experience of those units where a probe is inserted into the tailpipe. However, these are used on rolling roads to help tune cars and on this basis would have little doubt of their accuracy.
The problem with home use is that it is not possible to load the engine to measure the actual running AFR, all you are able to measure is the tick-over AFR which is normally rich.
I ask my local garage to run a measurement on my TC when I take it for its MOT. While it is not required for the test it does at least give me an annual check of my mixture.