If only 10% of ADSL users reach their cap, then there doesn't need to be a cap. If 10% use 90% of the bandwidth, that doesn't matter. It's the way the world works. Look at insurance for example. If you are a good risk, you are subsidising all the bad risk people with your premiums if you don't claim. Another example is air tickets. The earlier you buy a ticket, the cheaper it is. If you buy at the last minute, you pay a fortune and are subsidising the cheaper tickets sold weeks before. Everywhere in the consumer world, people are subsidising other's depending on the service. It makes sense and everyone is happy. Without it, prices would be rediculous.
With regards to P2P shaping, i was reading an interesting article showing that the problem with P2P is UPLOAD traffic more so than download traffic. Local ISP's should heavily shape UPLOAD traffic out of SA and that way literally double the available international bandwidth. This will not affect the speed of P2P gloabally, since each torrent has many peers, each supplying a trickle of bandwidth.