I used 10km torps on fubuki. 62 knots is much better than 57kts. And anyway, with upgrades and captains skills, you can make it invisible, rendering 15km torps useless (and you get higher chance to hit when closer ofc). Also, last hull just because of AA, while you lose one turret is not worthy in my opinion. Especially on Kagero, where turrets are made of plastic and tend to burn easily (even with main battery modification 1).