So Jinx's rockets having a mana cost that falls off in significance is actually intended. I prefer the decision making mechanism behind Jinx's weapon swap to be "What is the best gun for this situation?" instead of, "Am I worried about mana?" The reason the mana cost is there at all is to gate rockets a bit in lane where the low attack speed is much less of a downside. Mid-late game there's reasons to use minigun. In early lanes (especially sustain and poke matchups) the cost gets her playing the swapping game when she might normally just use rockets.
That said, I do feel like Jinx's early mana constraints might not be expressing themselves as much as they should.
Well then why not increase them? This really comes down to the question of where are the best places to buff or nerf a champion. In a world where you can set any champion's win rate between 45-55% purely by adjusting their base health you always have to look and ask, "While changelist A is a good nerf would change list B be even better?"
For Jinx specifically, IF we were going to nerf her (not saying we will or will not), I'd probably want to look at rocket DPS early in a fight. Jinx is intended to be kind of a force-multiplier sort of hyper carry. My vision is that you draft a damaged focused team with Jinx that all tries to contribute to getting that first kill ( so 1 support, 1 tank, 3 damage, as opposed to a protect the Kog comp that only defends with 3 supports, 1 tank, 1 damage). Once someone pops, Jinx gets excited and kills everything. WOOOO SNOWBALLS.
Right now it feels like rockets might want to be coming out slower, seems like it's a bit too easy for Jinx to get the first long range kill on her damage alone. Crazy idea. What if her Base AS on rockets was waaaaaaay lower, but as she Gets Excited! from kills her damage or AS starts to ramp and get stronger! Dunno, just spit-balling at this point.
Regardless, things are about to change up pretty heavily with the preseason and there's another part of me that feels Jinx's current strength is simply a product of the hyper-carry, double TP meta. In a world where any and all early aggression is heavily punished why WOULDN'T characters like Jinx and Vayne be taking over.