Re: OP
If you assume that a majority of customers will be on their Internet connections during peak hours, then the heavy downloaders have to compete with everyone else for bandwidth. Since heavy downloaders make up a small percentage of the customers, even if all of them stopped downloading during peak hours, the casual user will, at best, experience said small percentage increase in performance.
A rather crude example that illustrates my point: During peak hours, 100 customers share a 100Mbps connection, therefore, each customers gets (100 Mbps / 100 customers) 1Mbps if everyone is using their connection at the same time. Now if 5% of the 100 customers are heavy downloaders and they stopped using their Internet connection during peak hours, then each customer gets (100Mbps / 95 customers) 1.053 Mbps. That's an increase of 5.3% increase in speed.
Problem solved on the ISP's end.
/sarcasm