When you get down to the lace part of the hat you will be working over 86 stitches. You are supposed to get 17 sts=10cm if you get 17=9 cm here is what will happen. 17 goes into 86 five times with one more stitch left over, so call it 5. If you got 10cm every time you worked 17 sts you would end up with 50cm, 5X10. If you were only getting 9 cm every time you work 17 sts you would end up with 45 cm, 9X5=45. So that is not a lot and the pattern says the hat will stretch to fit a range of sizes so it would probably work.
I didn't notice how they worked the hat row gauge wise. I mean I didn't notice if it told you to only work a certain number of rows or if you went by inches or centimeters for the length. If it gave it in rows you may need to add a few more somewhere so it wouldn't be too short. Made from the top down it would be easy to try it on and see how much more you needed.