Geez even I get 7 towing at 70mph...
The 8.1 has O2 sensors before and after the cats (there are two cats, not just one) and the PCM is programmed to read and compare oxygen content before and after the cat to help trim fuel mixture. If you remove both cats, you will need to re-program the PCM so that it's not looking for those post-cat O2 readings. You may want to ask Black bear if they have done similar and what the results were.
As a side note, Banks Engineering is noted for their intake/exhaust upgrades, but they never made a kit for the 8.1 because the exhaust from the factory is already highly efficient. (They did make an exhaust kit for the 8.1 on the Workhorse chassis because the way that the exhaust was routed was inefficient, but that doesn't apply to GM-built vehicles with the 8.1.)
I did a lot of research on this, and removing the cats from the 8.1 is a lot of work IF you can find someone to re-program the PCM, and even then you may not see any improvement at all. I would say leave it alone.
I just went through the truck last year, all new stainless fuel lines, new fuel pumps (yes there are 2 in the Suburban), fuel filter, had the injectors ultrasonically cleaned and rebuilt, throttle body cleaning, new plugs and wires.
The result? Runs better than it ever did....but I still get 10 mpg around town and 7 towing!
