This will come off as very blunt, but I can guarantee you that you don't know nearly as much as you think you do. You may also not be nearly as smart as you think you are. It is incredibly arrogant to think you know already everything you could learn from undergrad at MIT or a similar school (Ivy league, Chicago, Stanford, etc). I'm currently a theory PhD student at a school with the same reputation as the ones you mention. I also chose between this school and Stanford. There are several students in my class who did their undergrad at MIT and they are all absolutely brilliant. The students at top grad programs were at the top of there undergrad program, that includes places like the ones you mentioned. These are the students you are competing with.
Most of the students in my class took several grad courses in undergrad, so even if you found the undergrad courses "too easy for you" (which I highly doubt) you can take all kinds of grad courses, even GR and QFT. A lot of the students at MIT started taking grad classes as early as sophomore or junior year. As an MIT student, you also have access to Harvard classes, so a lot of MIT students take QFT over there. Works the same for Harvard students.
Even if you test out of high school and go straight to a PhD, you most likely won't even be close to competitive for the places you mention because of your lack of research experience. You also won't have letters from well known people. These two factors are the most important component of a grad school application. These things are what make you stand out. I have friends at my program who have more than one publication from undergrad. I myself had a first author PRL and am currently finishing another first author paper from undergrad.
So my suggestion would be to gain some humility and see how everything turns out. Just try to get the most out of undergrad and go from there.
And one last thing, don't plan to go to one of those schools since you are in no way guaranteed to get in. I know brilliant students who were rejected by all of those schools, yet worked hard and got into all of them for grad school. It wasn't that they weren't smart enough to get in for undergrad, they very well could have been one of the most accomplished students there. It's just that admissions to these schools at the undergrad level are a complete crapshoot and there are a lot of other factors involved not related to your potential as a student.