The reason you should stick to the D&D core (in this case) is that at level 5 and a haste potion you can cast 4 fireballs in one round if you have 6 sorcery points and the circlet that give a bonus action with fire damage spells. I shouldn't need to explain why casting 4 fireballs in one round is ridiculous, even 2 would delete the goblin camp.
I do understand this makes spending 3 sorcery points on a quicken spell somewhat esoteric and pointless in most cases. I mean you could throw something and cast a fireball I guess. Haste doesn't allow two spells per round either. This is again something homebrew screwing up th action economy.
What would work pretty well with core rules is - haste pot, quicken spell, hex, scorching ray with the fire damage circlet, dual hand crossbows on a high dex drow sorc. Hand crowwbows would hit for a potential 2d6x3 then +6d6+3d6 from scorching ray. So 15d6 in one round at level 4 is pretty mean. Fireball x4 would hit for 32d6 to everything in the blast radius.
