The UK and European G10s don't have a THX mode either. The THX option is reserved for the V10 models, which are the next step up the Panasonic TV range. I find the best (and most stable) picture is offered using the 'cinema' mode: contrast at roughly 60%; Brightness at around 45%; colour at just under 50; and sharpness two clicks from the highest setting. Turn all other picture 'enhancement' features off -- noise reduction, and the intelligent frame creation option, which doesn't really seem to improve the picture, but can make motion look a little bit strange, on occasion.
Of course, different viewing conditions produce different results, but you can use a THX optimiser on, say, a Disney DVD/Blu Ray and see how you get on. Eventually you'll stop obsessing over tweaking the picture and enjoy watching what is an excellent television. 