Dave's linked chart is a good start but be aware, I think the data is wrong for the circuit on that chart. The prices are also wrong. I'm pretty sure the data was flip-flopped for the Catalyst and the Circuit...there is no way the catalyst's measured volume is smaller than the circuit's.
With that said, I would strongly recommend the Circuit for the JMT and the like. I had a similar base weight as you (12lbs with bear canister). Keep in mind, after resupplying at MTR, your total weight will likely rocket up to 30lbs because you'll be carrying that much more weight in food, unless you plan on going off trail and resupplying at some point after MTR. For me, the total weight before MTR always floated around 15-20lbs. I carried all this in a catalyst, which was probably a bit overkill, but much appreciated on the heavier days. But I had zero days of discomfort/hip bruising/sore shoulders--something unseen with my lighter packs.
I saw a ton of circuits on the JMT and everyone said they loved it. A few commented that they wished they had the catalyst because of volume. Specifically the Bear Canister is a beast of burden and takes up almost all the room making bag packing a task that requires a bit more attention/thought.
Personally, I think the support of the gorilla is a bit under-built for the task...especially at 30lbs. I'd rather suck up the extra oz and enjoy the cush of the circuit's suspension. The Ohm/circuit/catalyst all share what is probably the most comfortable UL hipbelt found on any cottage pack...this can be attested to by many who have put hundreds of miles on these designs.
While I'm positive a gorilla will get the job done, I don't think you'll be as comfortable as you would with a beefier pack. And on a thru-hike of something as amazing as the JMT, I would hate for you to be diverting any of your attention from the beautiful scenery because of slight pack discomfort. For that reason also, I would rule the CDT out. You will suffer with anything greater than 20lbs in a frameless pack for an extended period of time.
IMO, the better measurement of volume is to look at the main pack bag volume and ignore the pocket volumes. You can factor in the extension roll top, but keep in mind not all of it can be used because the drybag style top of the catalyst/circuit require you to roll the top at least 2-3 times for a proper seal.