Yeah but I figured I'll only need 5mm play to get the exhaust gaskets in and out.

The oil line 'gaskets' (as the service manual calls them) are just copper washers, right?

Will only undoing the down pipe at the top and not at the bottom give me enough play? The less joints disturbed the better.

Have you pulled your turbo off? No offence, but I can make guesses myself. Looking for the advice of someone who's done it.