How is a line line limited when itys "up to 40mb"?

I'm currently on a 75mb fibre package and actually get 45 - 50 mb down and 9 - 11mb up. As this is only slightly higher than the 40mb packages I was thinking of changing to save some money but I don't know how the throughput will be limited. Is it capped at a maximum of 40mb or will I get a reduced rate at 40/75 of the maximum my line can handle. In other words, given my existing line performance, if I change to up to 40mb will I actually get that or will it be more like 24 - 27?

Essentially it is all done via artificial caps so if someone gets more than 40Mb on the top package then they would get 40Mb on the 40Mb package. It is not a ratio of the package top speed.

Thanks very much.