How much does a good score on ACT/SAT increase my chances whereas I can waive them?

[skieurope]

But I know some of my friends have been accepted to Harvard, Stanford, and MIT with waiving ACT/SAT.

As a follow-up to my earlier comment about talking to the colleges directly, this is what Harvard says:

You are only exempt from this requirement if your country does not offer these tests and traveling to take them would be difficult.

https://college.harvard.edu/admissions/application-process/international-applicants

This is what Stanford says:

The SAT or the ACT is required for all undergraduate applicants. If no sittings of these tests are offered in your country, you may request a testing waiver. The request must be submitted by a school official in writing by email (credentials@stanford.edu), mail or fax: (+1) (650) 723-6050. Include your full legal name, school name and date of birth at the top of the letter. No form is required. Requests will be reviewed on a case-by-case basis.

https://admission.stanford.edu/apply/international/index.html

This is what MIT says:

Students from countries where the SAT and ACT are not offered for all students (such as Iran and the People’s Republic of China) will be considered without a full set of required test scores on a case-by-case basis.

http://mitadmissions.org/apply/international/intltests

That said, the odds are long. There are a grand total of 4 undergraduate students from Iran attending Harvard. Therefore, on average, they admit one person per year. Good luck.
http://www.hio.harvard.edu/statistics