There is no real difference between the HR-05 and the HR-05 IFX, except that the IFX is made for 80mm instead of 70mm fans (it's a little bigger).
I have the Ötzi on my NB because I too was scared that the base of the HR-05 would be too small for efficient cooling.
But a friend of mine uses it and gets excellent NB temperatures, so it's working all right (the NB chip itself is way smaller than the heatspreader, obviously).