You don't say what type/weight of sil-poly your tarp is made of. If it's a heavier weight fabric you could gain some space by going with a lighter fabric. And 12' x 8' is way overkill for all but the most inclement weather IMO. Especially the length. 11' is more than adequate for an 10.5-11' hammock.
You might also consider ditching the bladder and using 2 1-liter nalgene bottles (since weight isn't an issue for you) and carrying them in your hip-belt pockets. Or you could use Gatorade/SmartWater/LifeWater bottles if you decide to lighten the load a bit.
The poncho...carry this on the outside of the pack in the front mesh pocket or tied-up somewhere. FWIW, I bring all of the items you bring with the exception of the bladder and the poncho (I use a rain jacket) and it fits in my 38L Exos.
Edit
I just re-read your post. I'd imagine your Costco throws are pretty bulky in their stuff sacks. Since you're carrying those in a 6-mil garbage bag anyway you might consider losing the stuff sacks and packing the throws loose in the bottom of the bag and then everything else on top. This should free up at least a little space. A 6-mil garbage bag? 3-mil is more than adequate IMO. I've used a 2-mil the last few years with no issues. That includes 2 back-to-back fully immersed creek tumbles during winter.
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