If we give Ilyana a 1 weight thunder tome with whatever strength we want to attach to it, then what's to stop Soren from getting a max-strength wind tome as well?
A max-Mt Wind Tome forge is economic suicide (>15,000G). Soren prefers the Thunder Tome forge anyway. I don't know why there's any debate over whether Soren will be stronger or faster than Ilyana. He will.
Level 6 Soren: 9 Mag, 10 Spd, 0 Str
Level 6 Ilyana: 8 Mag, 9 Spd, 1 Str
Soren and Ilyana will be equally fast to start. Soren will be a little more powerful. Soren has a higher Mag and Spd growth (by 0.1 a piece). Clearly, if we forge our Mage a 1 Wt Tome, Soren will be leading Ilyana in Atk and Spd forever. But Ilyana does have a higher Str growth, so lets see what happens if our Mages have to make due with generic Tomes and Ilyana has some time to develop Str. (In the relevant stats (Str, Mag, and Spd) Soren starts with 99 Mag growth points and Ilyana starts with 50 Mag growth points.)
Level 14 Soren: 14 Mag, 13 Spd, 0 Str
Level 14 Ilyana: 12 Mag, 11 Spd, 3 Str
Behold! Ilyana has a potential 1 AS lead over Soren with heavier Tomes. So Ilyana can have 16 Atk and 11 AS with a Thunder Tome. But Soren can get 18 Atk and 11 AS with an Elwind. Or, if Soren needs the AS, he can use a Wind Tome for 16 Atk and 12 AS. And if Ilyana upgrades to Elthunder for 19 Atk and 8 AS, Soren can match that with Elfire (if he can wield it), or settle for Elwind's 18 Atk. And this is about as good as it gets for Ilyana. Soren is strictly better on offense (before even considering innate Adept). Ilyana's superior Str could theoretically be helpful when wielding siege tomes, but neither Ilyana nor Soren is going to be doubling with siege tomes anyway.