I have had an Autometer A/F ratio gauge in a supercharged V8 before and I agree that it's just a "light show" (ala KITT in Knight Rider ).
There has been talk that the Greddy A/F gauge is more accurate.
I actually decided not to buy a center gauge pod (which I think is a very elegant solution) because I couldn't think of 3 gauges I'd want to/was willing to run. My list topped out at boost & egt.
I know the only other thing that I wonder about is oil pressure, but I'm thinking the plumbing on that one might be a bit of a bear on the sending unit of that particular gauge.
Good luck with your search for a 3rd gauge! :-)