The worst part is that I don't even use the "grouping" feature. I have my taskbar set to "never group" with small icons. As such, there's never more than 1 preview window to select for each taskbar item, which makes it useless.
I can see how it would be useful if you used grouping, but I don't like the grouping at all.
In Explorer.exe, I did find:
... which also appears in aero.msstyles. I'm not sure where to go with that, though.