Hi Roger,
I do have a disk installed, the case only gets very slightly warm, (28C top of case, with CPU at 55C) so I don't forsee any heat related problems, the case is a big metal sink and radiator.
You need to remember that the CPU is on a board, which is slotted into a memory style connector, when you put the board in, it is inserted in the socket at an angle, say 30Deg to the main circuit board and then pressed down until it clicks in place, approximately parallel with the main circuit board, (see Gordons photo) so there is quite a lot of up/down 'spring' in the CM board, which will easily take up any thermal expansion, it will mean you can't make the heat sink to CPU thermal connection very tight, just touching, with thermal paste as a buffer, will be the way to go.
Regards, Kevin.