ask the factory
it's a natural convection heatsink
when mounted properly, air naturally flows between the fins and you get the "degC/W"
or thermal resistance, as specified by manufacturer.
since they are not mounted as expected by the manufacturer, they will NOT have the specified "degC/W",
as you know
asking for speculation on what will happen is pointless - ask the factory,
(the ebay listing shows the manufacturer is Aavid - they should be able to make an informed opinion)
or, take a chance, turn it on, and see how hot it gets