I am trying to steal weightless keys from members of the mages guild. Sneaking is at level 49 and I use a fortify sneak 60 plus enchantments to have the skill at about 150 just to be sure. It does not help much even if I pickpocket them while they are sleeping.

What else do I need for this? Higher agility? Does only the basic skill level count without the fortifications?

The thing you need here would be chameleon I′d say. It does not always work though, but at those occasions I tried pickpocketing a heavy sword from someone.
Snitching keys with 100% chameleon (or above just to be sure) should be no problem at all

If they're scripted to detect you, nothing will help. In effect, the key is notifying the holder that it's being taken, and nothing can prevent that.

I don't remember mages guild members having any specific keys in the vanilla game, is this a mod?

Yes, it's T.I.E., I am sorry, I was not aware about this:



I just want to get a certain key to open a display chest before it is lost (due to the mages quests). I guess I have to be expelled then.