I've played all three of those characters, and I don't think the cooldowns needed to be highered.

Shens is PAINFULLY long, especially if you do not have CDR.
Sorakas has an OK length.
Karthus's is another long cooldown that, when I was playing him, pissed me off.

So you are going to make Shens cooldown kill-yourself long, Sorakas longer than it needs to be, and Karthus's painfully long? OK, sure, whatever you say, riot.

Sounds like somebody is reliant on global ultimates to do well.

But IMO, Shen's ult should be nerfed to about Twisted Fate's ultimate's range without changing the cooldown. This would make him still a great champion, but not quite so hated by everyone who doesn't have him on their team.

Soraka DOES need some sort of nerf. Bot lane running heal with a Soraka post 6 is just ridiculous when trying to go all-out.

Karthus... Meh, Karthus will be Karthus. As long as they don't make it THAT much longer it isn't too big of a deal. It IS a strong ultimate afterall.