Interesting that you're running lean. The typical result of removing cats, is no oxygen sensor inputs, leading to richer fuel mixtures if no changes are made to the ECU program. Is your GT350 running a tuner/custom ECU program? I listed a few more conditions that can lead to lean mixtures in many vehicles, below (not all of these comments are meant to be modern GT-350 specific/unique). I'm assuming the engine is running smoothly with no power loss (lean mixtures can result in power loss among other things)?
Generally regardless of vehicle, when cats are removed you get an increase in horsepower (less back pressure), more fuel options (leaded/other racing fuels), lower engine operating temperatures (better exhaust flow), improved fuel consumption (but wait! Is that relevant to GT-350's (awarded a gas guzzler tax...
)), and louder (less-muffled) possibly improved exhaust note. There are drawbacks including EPA compliance smog cert upon sale (where required), CELs, possibly diminished low end torque, possibly increased fuel consumption if ECU program isn't corrected for richness (not in your case).
Some typical causes of of lean mixtures in other vehicles include clogged fuel filter, failing fuel pump, clogged fuel injectors, and (NOT in your case, because there are none) - bad O2 sensors.
Please keep the forum posted on your findings/outcomes. There are more then a few GT-350's out there w cat deletes.