I've personally never seen or heard of a waterproof spray that will protect against soaking. Water hitting the material might run off fairly well, but a continuous rain soaks it so much that water will just seep through. That might not be a problem for commuting, but I'm assuming that if you're going through the extra step of double water-proofing, you're probably touring. If you're touring, you're almost certainly going to be stuck riding in the rain for an extended period of time at some point.
Waterproof covers are ok. The problem is that they don't completely cover the pannier since they can't wrap around the mounting hardware. The back of the pannier will get wet. If there's enough rain, it'll get wet enough to soak through to what's inside. Another problem with covers, and this is the part that I like the least about them, is that it's just one more thing that you have to carry around. I'm not necessarily a weight-weenie, but I am a space-weenie and covers take up a fair amount of it.
Personally, I'd recommend skipping the dry bags (again, they just take up extra space) and getting fully waterproof roll-top panniers like the ones made by Ortlieb, Axiom, or Vaude. These bags will stand up to a freaking monsoon without any worry of water getting inside. The only real water related problem that you're likely to run into is that humidity can build up and condense inside the bags. But you're not talking about a huge amount of water there, certainly less than you would if you were caught in a downpour with a bag that wasn't waterproof to begin with.