This will work well, if you are going for a cache that is not found very often or if you download the GPS file often enough to have every log in it. If your caching buddy has found the cache a long time ago, they may or may not have a log in your download depending on when others found it.
I believe a manual GPX download will contain the last 20 logs while a PQ download should have something like the last 5 logs.
With this in mind if your planning a cache run in an area with commonly hit caches it is possible your buddy has a find that is not going to show up in your gsak updates.