A couple of thoughts from me:
first up is your choice of as5. You could do better by using one of the liquid metal type tim's .... Coollaboratory Liquid Pro for example.
second, I have heard of people having no clearance between the pcb and the ihs when trying to"lid". If you were to remove some of the material that is the very edges of the ihs you may find that the cpu to ihs interface is a tighter fit and therefore runs cooler.
The above image (courtesy of idontcare at anandtech) may make things clearer. you need to ensure that there is a gap shown in red as .14mm in the pic