In theory they now have pooled data plans, but in reality they're not cheaper so you probably don't care. Also, I've heard that a 3G PDP context costs money, so seven devices using 5GB total does cost more than one device using 5GB, although probably not as much as they charge.

we don't get a discount when the iOS devices use [Wi-Fi] for data as opposed to cellular

Sure you do; that data doesn't eat into your cellular cap. Effectively cellular is ~$6/GB and DSL is <$1/GB.