She got some bugfixes a little while back that made her E reset a bit faster. She scales very hard though, so going 20-1 would naturally make her incredibly strong.
mmmm... i played with a cassiopeia last night, she was casting extremely fast, and got an early quadra, carried the game at 20-1, i wondered if its a potential cheater or just a very skilled player
Increased safety and reliability, as well as more focus on her damage over time abilities.
She was recently reworked, but that sadly left her in a really bad state, and the buffs haven't arrived yet, so no, she wasn't buffed.