To me, what this card does is this:
"Sneakdoor" into R&D. This should help you get to 3+ points. When you or the corp enter the midgame, this card accelerates, allowing you to get even more agendas. This means that the more you steal, the more you get.
However, you have to remove the tag or else the corp will trash it. However, there are advantages to taking the tag, assuming you're using Sebastiao. Now you can slam “Pretty” Mary da Silva onto the table and then remove the tag. This should be a game-closing situation.
In summary, I think this is a really insane card that looks like a hybrid between Medium and Sneakdoor Beta.
My rating: 6/10? 7/10? can't tell how to rate it because it's hard to tell unless you play it.
Update: It's fine as is. Well balanced!
— DDDydra