I believe THX mode has more to do with color settings; not anything better in terms of deinterlacing or scaling with SD material.
I would honestly look at them side by side if possible, with some SD material. To me there was a very easily discernible difference in VP between the previous year's upper and lower line.
After choosing, you then you will want to find out how much more an outboard video processor would do for you. At the base minimum, I would go for a DVDO Edge, and from there the sky is the limit. The thing is I would be just a little afraid that the DVDO would give you marginal returns over the Panny's processing, but I really don't have a clue, and suggest you hit up more video oriented forums. OTOH, the Edge can apply EE or DNR if desired (I never do, but I only watch beautiful blurays).
However, I got to say it, you can only expect so much with SD. Crap in = crap out, and there's nothing you can do about that. Remember, with TV it's even worse since they compress significantly more.
