
All spells profit from attack upgrade. HT storm for example benefits from damage upgrade, or nova split missile, or snipe... and anything else too.
Yes, patch 16 will make the hero system completely different. You will be able to level your heroes and you will have special upgrades for heroes just as for your mercenaries and basic units. All spells will be re-made and upgraded / improved. New abilities will be added and of course, entire new heroes.