Doing 11 units in year 12 means you have one backup unit, which can count as one of the 10 units that will contribute to your ATAR, being your best 2 units of English as well as your best 8 units of other subjects. Where Mathematics Extension 1 counts towards your ATAR, the 2 unit subject in which you've performed the worst (except English) will count as 1 unit, allowing for Mathematics Extension 1 to count towards your ATAR and therefore halving the effect the 2 unit subject would have on your ATAR. This is what happened in my case, as Mathematics Advanced was my worst performing subject (final mark of 81). What ended up happening is that because I did 11 units, French Extension (a 1 unit subject in which I received an E4 as my final mark) ended up counting towards my ATAR, halving the effect Mathematics Advanced had on my ATAR.
As mentioned above, if Mathematics Extension 1 is your worst performing subject, your best 2 units of English as well as the other 8 units (excluding Mathematics Extension 1) will count towards your ATAR, meaning that, in such a case, Mathematics Extension 1 will not impact your ATAR.
I hope this helps!