So you have not been disappointed with service being on a lower tier? No issues?
In terms of the QCI value, most AT&T postpaid plans (except Elite/Premium) are still the same as most prepaid plans (the exception is the cheaper unlimited plan). Since you're not on Elite/Premium, you'd have the same experience as your current plan. In most cases where AT&T has good service (capacity to the tower and a lot of spectrum deployed), even on a lower QCI plan (like the regular Cricket Unlimited Core plan), things are fine enough you won't notice (I say this having used AT&T postpaid, AT&T prepaid, Cricket, and an AT&T Business plan at work in the same area).
As others have said, there is some push-pull and it seems AT&T has at least 3 tiers. You're in the middle right now and would stay there if you went with a capped plan (5GB, 8GB, 15GB, 16GB).