I don't have any 7* yet, at lvl 63. I have several 6* instead, on purpose. I don't know if it's related to it, but my GW are cakewalks. I never see teams of 7* toons, and I'm top 5 regulsrly at my timetable. I've sniped 2nd place three times.
I know I'll have to get 7* soon, it's already hard to keep the place in top 5 arena, but so far I think my approach is working well. Maybe the GW matchmaking is totally unrelated and I'm just a victim of confirmation bias, but it has been working. I clear GW at lvl 63 without losing a single guy. I use Lumi, JC, Geo, 86 and 88 the first 9 or so battles to do it faster, and swap Daka for 88 last 3 to clean it up safer. And it's a cakewalk. Dunno if others' experiences are the same without 7*