How much do Grade 9 and Grade 10 marks matter when applying to the US?

Yup.

To put things in perspective, even with perfect or near perfect stats (close to a 99/4.0 unweighted average and a 1600 SAT) an unhooked international applicant probably has less than a 1 in 40 chance (less than half the overall acceptance rate) at schools like Stanford, Harvard, Yale, etc.