if your talking airflow it would be more important to cap the edges of the HS to force the airflow through more of the central fins instead of shortcutting through the edges right? And even with the typical heatsink in a push pull configuration, your getting hot air on one side of the heatpipes anyways, so using a fan in the center the encourages airflow through the higher resistance central area of heatsink, doesn't sound like a poor idea. Besides, dimpled fins, and i think with holes in the dimples, have been shown in a couple papers to give the best internal air turbulance for better cooling, so even then, this isn't doing the best you possibly could