Hi,
I would recommend that you do this reclamation after business hours when there is minimum downtime, also note that you are using ESXi 5.0 and there were some issues that were reported w.r.t. to Storage performance hence this option was disabled in the next ESXi update.
I have the UNMAP primitive for ESXi 5.5 and I see that the controller CPU spikes to 20-40% depending on the existing load, hence suggest that you test this only when there is minimum load on the controller and ensure that you have backups.
I have written a blog specifically about ESXi 5.5 and UNMAP primitive with NetApp, you may refer to this blog post or other vStorage API blog posts for more information.
Data Center on Cloud: Using VAAI UNMAP on vSphere 5.5 & NetApp Storage
-Arun
http://highoncloud.blogspot.in/
About VMware Virtualization on NetApp