If it's a feature for the mod queue, I doubt the load would be enough to cause any trouble, if it's even noticable. I certainly wouldn't mind to test it if you like.
Currently the search is being used about 10-20 times per hour... even with a link on the upload page I doubt the server would break a sweat.
And even for a "find duplicates/similar pics" link on each post page would be manageable with some caching, I think. Though I'm not sure that would generally be useful.