Don't change the thermostat. The best fix for the trans temp is to install a stand-alone trans cooler and get the radiator out of the mix entirely. Plenty of threads about trans coolers here and other GP/LS4 sites that discuss this very issue. Tripleedgeperformance.com has a kit for sale to do this. You can do it cheaper by gathering the parts yourself, but Dave @TEP has everything you need all set in his kit.
I wouldn't recommend changing to a lower temp thermo. Remember we have aluminum V8's, and aluminum will expand/contract more with temps than iron blocks will.
