I've been playing around with the Spears and Muncil test disc on my 60gt30 and have discovered that in THX mode peak whites are clipped to some extent at anywhere above a contrast setting of 46. This doesn't seem to be the case in several of the other modes i.e. Game, Custom, Vivid. The clipping appears to involve about the top 6-8 steps in peak whites in the "dynamic range high" test pattern. Standard, THX, and Cinema all appear to behave similarly while Game, Custom, and Vivid appear to have a greater dynamic range available. Source is a ps3 via HDMI with all standard video range settings applied.
As you may know, in real world viewing, this loss of peak white info can cause subtle loss in high white details depending on program material.
I'm experimenting with Custom mode at this point with Low panel brightness as it seems to be adjustable to closer to how THX looks overall (at least to my eye).
The other option would be to stay in THX mode but drop the contrast setting to 46 or lower--although to me this is a little subdued.
It's notable that I'm using factory default SM settings (have not altered the SM).