Moonstone stone renewer seems troll on Gwen, since it give barely any AP. That's not true though. If you compare it with Riftmaker, it's easy to see that the AP that you are not buying is heavily compensated buy the saved gold. The aforementioned gold can then be invested in the Staff of flowing water, a 2300 gold item that, thanks to the synergy with Moonstone renewer, can give up to 95 AP.

Mejai's soulstealer is one of the most misunderstood items in the game. Most people only buy it with an almost full dark seal but Mejai's shows its true potential at low stack. This items is great for comeback since it increases your gain from random kills or assists while you basically aren't risking anything: Low risks and high rewards.