Hmm, I think you may have found the common thread, MeatBeef. Drat.
In most cases, though, isn't SSE support a compiler-level thing? Can't you just disable it when building through Visual Studio? They might have simply left it on from the Xbox builds... which means they could issue a patch or a new binary.
Besides, I'd be quite surprised if the coders at DoubleFine were hardcore enough (or desperate enough for performance improvements) to have done custom assembly routines for rendering and physics etc.
(Not to impugn the skill of the coders in any way, of course, it's just that these days there's so little benefit in doing so, especially for the Xbox's PC-equivalent architecture.)