Well, since nobody else posted, I started a game, cheated with the World Builder to give my starting city a citadel, and gave myself Economics and Construction. Then, I started on a catapult. When the catapult was finished, I did not receive any experience points--it was a completley green unit. Therefore, I am forced to conclude that the experience point bonus goes obsolete as well at Economics.
Personally, this strikes me as a design flaw. I realize that many people here think Isabella's traits are not too strong (and personally I would rather have a number of different combinations), and the UU strikes me as quite useful: a knight with +50% vs. melee (pikes won't be able to take this sucker easily) and it receives defense bonuses? That's a pretty awesome UU!
But the incredibly limited focus of the UB...I don't know. Maybe mass-producing whatever siege units you have now and upgrading them is the best course of action--you are be able to squeeze 3 promotions out with civics (+4), barracks (+3), citadel (+2), and a GG (+2). Sure, you lose 1 XP in the upgrade to stuff like cannons and artillery, but at least you have the 3 initial promotions.