If by source you mean current win rates, you can look at any website like lolking or op.gg. If you mean win rates before/after snowball, not sure if you can find that. I only have my words to back me up hah. I have been playing ARAM ever since it was a thing in custom games back then and before Riot stepped in with an official queue, I'm kinda familiar with the map. I remember looking at win rates now and then before taking over one year break from the game, I only came back earlier this year.
The only champions that benefited from snowball are champions without innate dash to begin with, but with actual killing power, so when they finally dash in with snowball, stuff happens. IE= R magnet: Talon, Fiddlesticks, Galio, Kennen, Alistar, Karthus.. and half of these are still very mediocre today, simply because dashing in, now and then is not enough to win games. You need the individual strengths I described above.
Maokai has always been strong, and Sion, snowball happened around the time he got his rework so I can't really tell, but it's most likely the same as Maokai, press R > stick in > be forever annoying and kill stuff. These 2 champions are only strong because they counter the current meta dominated by ranged champions. Both are a pain to deal with as a ranged champion, they are sticky, beefy and deal lotta damage, with or without snowball.