I hardly think that ARAM is the same as SR and TT.
ARAM is very literally one of three situations... siege, dive, or some combination of both.
Many champions play exceedingly different on ARAM than on SR or TT.
One easy example is Shaco. Building either AD or Tank on Shaco in ARAM more often than not turns out poorly. The more common Shaco build in ARAMs is AP. AP Shaco on SR is rather useless, because unlike ARAM, enemies aren't forced to funnel in through a single lane, and as such, not forced into your jack-in-the-boxes.
Another example is Nasus. the standard Q-based Nasus on ARAM is utterly useless, because it's not possible to get a useful number of Q stacks. The AP build is far more capable. While on SR and TT, AP Nasus falls well short of what Q-based Nasus is capable of.
"Mastering" how a champion plays in ARAM will help you very little in SR and TT. Sure, you might have good mechanics for that champ... but mechanics mean very little when you overaggress against a champion that has a significantly stronger level 3 than you.
On top of it, there is no jungle, and no laning phase in ARAM. A lot of "mastery" for champions and how to play them, is to know when to play aggressive and to know when to play passively in lane. What matchups do you have to be more passive about? What do you need to be more aggressive against? What levels do you get exponentially stronger? What levels does your enemy? What do you need to build in laning phase vs teamfights?
None of that analysis exists in ARAM. ARAM is constant aggression and constant teamfights. And as for teamfights, even that has been simplified. Teamfights in ARAM just consists of the frontline, and backline. There's no sides, flanking, picks, or any real tactics. It's literally "which team can unload their kit on the correct targets first?" Target priority is pretty much the only thing that matters in ARAM, which is only one small portion of teamfights in SR and TT.
Don't get me wrong. I don't dislike ARAM. I do enjoy it from time to time. But I learn absolutely nothing about the champions I play from playing an ARAM.