Thanks for your answer!!! It finally confirms what I feared when I saw most people only talked about pumpkins being doubled. It is not only in your case. I think this is the way it is working for everyone. No one names the other currencies, though I also think ghost essence and creep tokens are seasonal currencies too. That's what it says when you click in their total number in the list of items you can buy with them.
I won't buy the booster again. I think I'll have more pumpkins than needed and more than the ones I can convert. For what I've read from people who got the not-working booster (there are many cases in several forums), there are some cities where it works and some where it doesn't. And no matter the number of times you buy it (a player tried 10 times), if it doesn't work in one city, it won't work in any try. It took me two days complaining to EA, but as they don't answer I finally got my money back from Google payments. I won't repeat the quest...
At least I know now it wouldn't give much advantatge. The real difference would be doubling the converters' ratios. But of course EA won't risk their incomes giving such improvement....
Thanks again for your answer 😺