That's a big stout. I always assume my dry yeast is 100% viable, and I always use one packet for a 5-gallon batch below 1.080 or so. You could use 2 packets if you're worried, but that would be overpitching. I've made several 1.07x IPAs with this method, rehydrating or not, and always got good attenuation. Even the 8 gallon IPA I did a week ago only got one packet of Nottingham, and it took off just fine, though I should have gone with two packs for that one (or I should invest in a larger package of yeast and measure out what I need, say 16 grams). I've never had off-flavors or incomplete fermentations, but I've only been at it for a little over a year.
I've never repitched at bottling either, but I also don't secondary. My rule-of-thumb is treat everything under 1.080 the same. Not saying that's always correct, but I'm giving my yeast a lot more credit than I used to. I'm sure somebody will disagree with such flippancy, but I would say at least in terms of the viability of dry yeast you shouldn't worry about it, especially over a year out from the "good until" date.
Also, 8 gallons is 30 liters, not 20.