Yes and no. When running lean, there is a much higher probability of mis-fire. If the mixture does indeed light off, it will burn very hot and clean (apart from high nitrogen oxides). But the high probability of misfire tends to kinda offset the gains.
Being a Chrysler guy and having lived through the "Lean Burn" years, I can say that the basic idea is sound enough but implementation is a bear (and especially bad when trying to do it with a carburetor instead of EFI). Lean-tuned engines will turn in very good fuel economy, and it can be done without sacrificing too much performance IF you can enrich sufficiently on demand. But it puts a lot of stress on the mechanical side of things. Combustion chamber temperatures get sky-high, exhaust valve temperatures get high (although downstream exhaust temperatures may go down because there's less "tail burning" outside the exhaust valve). Pre-ignition pinging is just a way of life, and the ignition system has to be in perfect working order to successfully light off the lean mixture all the time. Once a system like that gets some age on it, it will simply not work as well as a "normal" system at a similar age and with similar maintenance. That's why just about any Lean Burn Mopar still on the road today has had a regular carburetor and ignition system swapped in place of the original Lean Burn computer.