Enchanting will help you greatly. Since souls are not in a shortage to get, the soul siphon perk is probably not a good investment. In order to get really good enchants on your clothes (like the fortify destruction or some other school), you will need to use the highest level gem and have the Enchanter perk maxed out with the Insightful Enchanter perk. At that point, you can make 4 pieces of gear have a fortify school enchantment and cast those spells from that school for free (-25% each). With the dual enchants perk, you can do this with two schools, meaning that you can cast almost half of the spells in the game for free. At high levels, this becomes essential for a mage character, as your spells don't scale up in damage like weapons do... you need to be able to dish out damage
longer, because you can't dish it out
harder -- if that makes any sense.

-Loth