Does 1Gb/s service slow down to the same speed as 300Mb/s service when the network is congested?

I currently have 300Mb/s service and am getting around 300Mb/s, or slightly higher, during the middle of the day. At night it slows down considerably, to around 60Mb/s, sometimes less, and I sometimes see buffering on the TV.

I'm wondering if I get 1Gb/s service will the service slow by the same percentage (80%) to around 200Mb/s (which would be acceptable) or will it slow to the same 60Mb/s that I'm now seeing?