I opened up the fifth factory slot the other day. Not a single regret, because now my base is completely finished. There's always the option to spend cores in the store later if the events don't suck them all up, and all the while I'll still be cranking out new tanks in the factory for basically free...in fact, I just put two 4*s into production while I was composing this post.

The store is a fool's bet for FTPers, even at the 'generous' 3% feature rate.