My kids are at an IB Diploma school and when my daughter was applying to colleges a few years ago we were definitely told that doing the full IB Diploma was looked upon favorably for admission. It needed to be the full IB Diploma, though. The IB certificate candidates are not looked at in the same manner. Colleges not only understand the rigor of the program, but also know that students have been in a well-rounded curriculum of subjects (and TOK and done the EE).
The IB Diploma is graded 1-7, with a 7 being essentially equivalent to an A+ and a 6 equivalent to an A. You have to remember that not all schools in the US actually use letter grading systems and colleges/universities know this and are prepared to deal with it. There are some high schools that don't even give grades and still get students into ivies and top tier schools so I wouldn't worry about the number vs. letter grading systems.
You're right that getting all 7's is very difficult in the IB Diploma and, in my experience, colleges/universities do understand this.
Watch the videos below with the Asst Admissions Director at Stanford University talk about how they view the IB:
https://vimeo.com/163260155
https://vimeo.com/163260154
https://vimeo.com/163260153
(You might need to copy and paste the link into your browser. I'm not sure I added the link properly...)
Good luck!