Suppose you were on 40/10 and at peak times the speed dropped to 20Mbps. If you decided to pay more to upgrade to 80/20 to get getter speeds and found that at peak times (just when you wanted to use it) the speed still dropped to 20Mbps would you be happy?

That's the point I'm making.

I'm just hoping that like the upgrade from Max to Max Premium gave higher priority on the BT network and often helped on contended exchanges, upgrading to 80/20 gives better speeds during peak time as well as off-peak. Is that unreasonable?

See another post in this thread, you get better priority over the Openreach network

Now what your ISP does is another matter, but then the same applied with Max versus Max Premium before