At least in NA, from what I can tell, nearly everyone dropped exactly one tier. My two Plat friends landed G5, all of my silver 4/5 friends landed bronze 4/5, and my gold 1 friend landed silver 1. I was silver 2 and landed bronze 2, but I climbed to bronze 1 in a day before taking a break by duoing with one of my former silver 5 friends.
I don't really feel like playing a lot of ranked, though, since it's bizzaro-land as high bronze/low silver is littered with everything from season 4 bronzes to season 4 golds, so most of the games I didn't duo in (since they bring the MMR lower than most former golds) are complete shitshows, either hilariously easy wins or frustratingly massive losses. Second game I play, my team is composed entirely of actual bronze players, the other team has a mixture of silvers and golds, by 9 minutes I'm 1/1/0 (after going for a suicide attack when I get caught in an inescapable gank) but the overall score is 3-18 for the other team. Fourth game, my team has a former gold 3 ADC, gold 2 jungler, and gold 4 top, I go 2/0/0 in lane when suddenly everyone roams over because I'm technically the "losing-est" lane and the game ends as an absolute stomp of something like 39-7.
Anyhow, playing Kassadin is hilariously straightforward in Bronze (assuming you get an actual bronze lane opponent and jungler). His weaker laning phase isn't punished really heavily and you can actually get a head up with a kill or two just through using his poke in conjunction with his shield to get good trades. Half the time the enemy team doesn't expect a gank from mid, and Kassadin is incredibly good at securing kills so you can snowball incredibly fast even if you don't totally dominate lane. I find that I can die between 1 and 3 times in lane or on ganks, but so long as I pick up a few kills during that time I will hit the "Kassadin Singularity", at which point Kassadin can jump on a semi-squishy target (say, an ADC during a botlane gank), delete them immediately, eat whatever damage is given in retaliation, and then jump back again, possibly while spamming /laugh. Once this singularity is reached, unless the enemy teamcomp is particularly anti-Kassadin (as in, it has a whole bunch of bruisers with gapclosers/hard CC and no super squishy targets), Kassadin will not die again and will rake up a massive KDA regardless of the performance of his teammates. He just scales so damn well, and since the changes to his ult he doesn't really benefit from going Archangel's, so he hits his powerspike much earlier.