I've been playing this since before he was hit by patch 5.4, now I play it more than ever. A build using Muramana and Iceborne Gauntlet as core components of his damage, sticking to people forever with Riftwalks as you never run out of mana, and bursting people down with that Muramana damage.
I consider 5.4 a buff, and this build is why.
Please enjoy it if you try it, and feel free to comment on the guide or ask questions/give criticism. :)
EDIT: I would show a video to demonstrate the build/playstyle since it differs a fair amount from typical AP Kassadin most people are used to, but unfortunately LoLReplay is annoyingly stupid and only seems to record custom games. :s