Would upgrading to a faster broadband make a difference if it's already slower than advertised?

Basically we're on an ADSL 8Mb tariff, but at best get 5Mb (and our speed decreases during the evening presumably because of other people in the village on the same line). I understand that nobody ever gets the maximum advertised speed, but am wondering if upgrading to a faster tariff would improve my speed even at times when the line is bottlenecked? Or if I'm on at times when my speed usually slows to 1Mb would that always be the case regardless of whether I'm on a 8Mb or 20Mb tariff? Thanks guys! :-)