The extra heatsink might be a good idea but where does it go , that thing has to take up space somewhere and it would make the card take up a lot of space. Are you planning on extreme overclocking that you would need extra heatsinks? It seems to me that three fans would be sufficient to cool the card under full load. If you look at the picture of the Artec Accelero Xtreme plus 11 the backside shows this massive heatsink and combined with the three fans will dissapate 300w of heat.
If you go to the site you can see the accesories they have there.http://www.arctic.ac/en/p/accessories/116/vr003.htmlhttp://www.arctic.ac/en/p/accessories/115/vr002.html