Ah, you bring up a good point. But I have thought of this.

To better explain the balance, I will go into how magic in this RP works for a bit. Let's go back to guns. Magic itself is very much like a bullet. It cannot be used without a cost, nor can it be fired without risk when not used in a proper weapon. In this world, the more powerful a magic is, the more immense strain it inflicts on the user, hence why END is important for spell-casters. Without high END, a high SPI user could wind up a super powerful spell, cataclysmic even, but it would have a [i]very[/i] long casting-length, because while they have great control, anymore stress to speed it up would literally disintegrate their body. So even a character with super high SPI is left to fire a few so-so ones before their body, which can viewed as a 'medium', tires out. 

I mean they [i]could[/i] just overload their body and blow themselves and their opponent up. But who really wins at that point... 

Now items might change this here, and there... but now we're just going into the realm of speculation. XD