Eh. GW is being nerfed down to 40% and the loss of 20 AP on Zhonya's is being put back onto RoA. Gold costs are all going up, but ambient gold is being increase on top of starting gold increased by about 1 per 10s, so 6 per minute and blue buff gives a small AP boost on top of the mana regen, plus the new flasks will make up for the mana pots on top of him having about 50 more base mana. Considering he gets 10 mana per minion kill, his mana problems early on won't be too bad. R will always be a pain, but less so.
Honestly though, the only thing Swain actually will have a problem with is if his enemy is AD and get an early executioner, but even then in lane, he can typically bully most AD champs out of mid and top on top of most of them not wanting to use that gold on item that doesn't build into anything useful until mid to late game. Normally a LW is good at all points of the game, but if Swain only has 54 bonus (45 from Zhonya's, 9 from runes), the at 40-47%, they'd only be ignoring about 20 or so of it, which for that, they can get a Youmuu's! lol.