I've applied to US and been through the entire hell(Seriously, it's way harder than the UK applications with the essays, recommendations & special stuff you need to send in).
I've also had the privilege of mixing with 2 successful Ivy-league applicants (Princeton & Wharton Business School), so I think I have a clear idea of applying to US schools.
What it takes to get into the top-tier US universities isn't academic results, but it also isn't just ECAs either. What it takes is CHARACTER & PERSONALITY, knowing who you are, what you want out of life and displaying that personality through your written essays and recommendations. All your previous achievements and successes have to somehow fit into that mould of a person you intend to project to the admissions counselors and they have to see that clearly in you.
That's the most succinct way I can summarise the US applications process.
Now, regarding applying for transfer to MIT, IT'S TOUGH. Their freshman admission rate is less than 10%, but their transfer admission rate is probably less than 5%.
You gotta be some crazy whizkid who wins international physics competitions, or won some Young Researcher Award to get in there. I'm not joking. That's the kind of students they're looking for. (This is based on the profiles of some MIT students I've read about.)
My advice is, if you intend to go MIT, apply there for a Master's, unless of course, you're really intent on transferring there. The US application process really makes you grow as a person, and irregardless of the final result of the application, I have no doubt you'll come out a better person. (As it did for me.)
p.s. You must take SATs. Elite schools aren't known to make exceptions.