Admin & Ultimate SD damage has been buffed to become higher than Mega spells but they have an exhaustion of two seconds (Admin sd is 5 hits while Ultimate SD only 3).
While they didn't change the Mega Spells.
Before the change, The Mega spells were actually better than the SD. (Perhaps not in the lower rebirths). Also from my own experience, you deal more damage with and SD if you get an higher rebirth and Magic Level. There's several SD's in quests you can use (even though I'd personally use Mega spell up to rebirth 750).
Also after talking with Admin some weeks ago, he confirmed that it's now easier to level a low rebirth up to a rebirth I currently cannot remember. Might've been up to 700-800.
Point is that this is not a huge loss.
If we go back before this change, you'd as a rebirth 350 Sorcerer, deal more damage with Mega Vis than Admin SD. (Have in mind that they haven't done anything with the Mega Spells).
And you can also do 2-3 Mega Vis before you cast 2 SD's.
I'd personally be glad that you don't have to donate for an item in the lower rebirths.
I doubt you as a rebirth 350 go and try to kill rebirth 1000s. (the players that use the Admin SD).