Expand would suggest multiplying to me.
I think "Add 15/x to all pendulum and pillar stacks" would be better wording. Expand is ambiguous, and, as I said, seemed more to suggest multiplying, rather than adding.
As for the card itself, I don't think it would be very useful in many decks.
To take advantage, it would have to be used early AND you would have to have more pillar stacks than your opponent.
It would take a very specific deck to really take advantage of the card, since most rainbows will have 1 or 2 stacks. And if you can't get enough gravity early, the bonus doesn't mean much. If your opponent has as many or more stacks than you, its bad to use.
Just imagine if you are running mono gravity, trying to get this out early to win fast, and your opponent is running fire/earth.
You may have gravity pillars and pendulums. They may have fire pillars, earth pillars, and pendulums of both. No matter when you played this card, assuming at least one of each of the sources of quanta was already out (quite likely given the cost), you will get 30/X (40/X for the upped) extra quanta generating permanents. Your opponent would get 60/X (80/X for the upped) extra quanta generating permanents, making it a poor idea to use it at all.
If its not in a mono/duo deck, it will likely be too slow to use to be worth making space for it, when you could toss in another pillar that could be played at any time, without needing to save up a ton of gravity quanta. It would likely be faster than getting one or two extra pillars/pendulums on every stack, anyway.
Can you give some example of how this card might be used without hurting you more than helping most of the time?