I did some quick testing, here are the results:
Char lvl 105, most suggested early game gear, all items mo'd fully, which is a generous assumption already, all lvl 105 charms.
Without tiefling dex: 2151 dex, 3590 average maelstrom dmg
With tiefling dex: 2656 dex, 4228 average maelstrom dmg
This is a loss of 9% block chance, and ~15% dmg.
Yes, yes, you can come with the argument that you can use ways now. Way of the phoenix adds ~700 fire dmg to weapon at this lvl, with a total of 35 pierce. This means, on bosses it adds roughly 700*0.65= 420 dmg per hit, and that you will do roughly the same damage. Add doom into the calculation, and the pure magic dmg from maelstrom wins. And i will just mention the fact that later bosses are balanced around having -100 resists (which means 170 pierce), so ways will fall behind greatly as you level up, because this build cant afford to stack pierce.
Unbuffed stats: