Master spells?
You mean the ones with the 3+ second cast time that you can't get off in combat without abusing the AI or having a meat shield that will get hit by the spell when it goes off?.
The one that does a blazing 100 (150 perked) damage in an AoE for 1200 Magicka?
So you didnt get them (probably newer played mage either) and your talking [censored] about things you dont even know off
Fire Storm 0007a82b A 150 point fiery explosion centered on the caster. Does more damage to closer targets. 1222 Complete Destruction Ritual Spell Master Master Cast on Self
If you got the master perk and got 100 skill level then without any other spell reducton cost gear or pots the cost is only (1222x0,4)=488 mana and the dmg while perked is (150x1,5)=225 dmg. Your forgetting that fire storm is an AOE spell thats gonna hit numerous enemies for 225 dmg and do extra dmg while they burn at the same time.
Still your choosing the most mana eating spell with a long time to cast. While not use chain lightning that is more mana efficient, hits 3 target at the same time, can be cast while moving and stagger enemies. But ok lets choose a different master spell like lightning storm which does 112 dmg oer second for 47 mana if you got only 100 skill and perked master in destro.
You can do 112 dmg per second for as long as your mana bar isnt zero. So if your lvl 40 with 100 skill in destro and mastery in it and your running naked then you will have 500 mana ( didnt add any racial bonus or magica gear ) so you will be able to do 112dmg for 10 sec so 1112 dmg total. Thats enough to kill a Draug Deathlord on Master level.
Still not satisfied ? Yeah I guess after all meele needs to smith the best weapon, upgrade it, enchant it, enchant your gear with meele %dmg to be able to kill things. So why not use the same for a mage ?!? Drink your custom %dmg pot and poison your enemie - now you are doing 112x2,62x2=586 dmg a sec for 47 mana.
You destroy a Draug Deathlord in 2 sec, you destroy a blood dragon in 6-7 seconds and that doesnt even take into consideration the disintegrate perk ( kills enemy that goes lower than 15-20% his health in one hit) and any less magica gear or even mage robes.