I think that currrently, in these dev machines, Apple implemented TC in Rosetta to make sure that people won't succeed in what we are doing right now, trying to run the GUI on non-Apple hardware.
And of course there would be performance penalty in the GUI so the performance of the current Tiger intel is still a bit far from the real Mactels we will see next year(if the first releases are projected next year). The performance of the dev machines are not so important since they are for developers to develop mactel enabled programs beforehand, and Apple strictly announced that no performance report should be done using these machines. I believe that real mactels will have native implementations of the GUI, and maybe a more strict TCPA lock on some other crucial programs to prevent the retail version of Mac OS X for mactel to be run on non-Apple hardware.
These are my own opinions of course.