How much extra fuel?

[alloneword]

Being a thick ATCO I was wondering if someone could help with these two questions:

1. How much extra fuel would a typical "charter" jet, eg, A320/B757 burn if it were held down after departure at, say, FL200, for about 150 miles when it could have normally expected continuous climb to at least FL310?

2. If there is a fuel penalty to operate as above, would it be a similar amount of extra fuel on the return leg if same aircraft had to decend from say, FL370, to FL200 about 100 miles or so before normal?

Thanks.