A properly made threaded connection is as watertight as any other method. Depending on what's on the panel, since it likely has both hot and cold connections, you can't have threaded connections on both ends, since there's no way to rotate the panel. There are some threaded connections that would work, though. The ID of the pipe is the same whether it is a threaded nipple or a soldered pipe connection. What's best for this situation would be determined by exactly how the thing was designed, and can't see that.