It only runs rich when you shift gears or let off. Last time I checked I don't accelerate when I LET OFF the throttle, thus running richer would not affect anything to do with performance. I guess you may get a couple more backfires due to the richness, but other than that I cannot see any other cons.
I already have a CEL for running the 3" MAF and no cats, so that doesn't really bother me. Although I have never had to clear (or heard of) a CEL for "running rich"...
Running rich won't effect your performance...but it will eventually gum up your injectors and spark plugs...oh..and by the way the CEL code for system to rich is P2188 -Bank1....so run your TBV how you like..just trying to save some people extra work changing out misfiring plugs n' such..