ok so to update what ive found out so far....

there is no sweet spot. ive asked around on my server people who are at various levels ranging from 70-112 what they are getting in arena chests atm... i have been told there are people lvl 70 getting sirius mats and yet others at lvl 90 who only getting justice mats... i did find a guy who was getting crosses until he leveled up to 72.. and another who is currently getting them drop and is level 96.

I have also checked on a couple other servers and on beta too and results above are not unique. maybe im missing something here, but i cant see where it is possible to determine what mats drop in a particular range of levels. if anyone else has tried or is currently looking at this please comment if same has been true for you also.