ive played dexxer a long time now and from what i've experienced, the hit spell is not affected by SDI at all.
however i think it is slightly affected by damage increase. On the weps with less damage ive noticed that lightning, say if you hit for 8, then lightning will hit for 8 too. But if you use a high damage weapon and hit for 20 then the lightning seems to do more damage. But i think it's capped at like 12 damage or something.
i might be wrong, and it might have just beeb determined by this certain players resist on his suit.
But no, SDI makes no difference.