I don´t cross cut the bottoms and, obviously, it depends on the size of the sprouts. Maybe 15 is a bit too long; I´ll always test them with a toothpick before draining them and they DO need to be slightly "crunchy".
I cook them 5 and a half mins in the microwave or around the same time as you (15 mins) if boiled. They aren't 'mushy' but do have some softness and a crispness too. Not sure if that cross in the base makes any difference at all!