Yes, it will decrease, but it is not precise mechanism, tcp will keep trying to increase data rate again and again. TCP bandwidth estimation policy is very complex and differs on each server (depends on server settings), it might take in consideration latency, ECN and other things. Read:
https://en.wikipedia.org/wiki/Category:TCP_congestion_control (p.s. ECN often ignored)
There is QoS devices that works with adjusting tcp window very smart way, such as Packeteer, who does interfere in tcp flows and adjust window size depends on bandwidth utilization, those are much more efficient in preventing drops and quota losses. But if you do usual QoS(not smart one like packeteer) on your side, it wont be much better in matter of losses, than shaper or DSL limitation on ISP side.
And still ANY dropped data is accounted at ISP as consumed, and more you drop - more you lose. If someone in your network will use torrent or many "flows" downloader, ratio might be that bad, that 1Gbyte downloaded, in ISP will be accounted as 1.5GB.