How about a monthly "on-peak" cap, but un-metered off-peak? Say 100GB/month on-peak cap so you can download at any time until you reach that cap, then you're limited via STM to the lower limit until its off-peak times? Would that work better than an overall cap? I think it would some how since then if you really want something, you can have it, or if you're not fussed you can schedule to download it out of the peak time cap
Then there is the argument over what time is peak time too.