Thanks for you trail-specific feedback, Matt & Jack. I wore my new OR Helium Rain Jacket (200g, 7.1 oz in size large) into the shower this afternoon, stood with the water spraying pretty hard, and gained some confidence from that.
Yes, the DWR is new, yes I wasn't under driving rain for an extended period, but as everyone knows, at some point it's not about "staying dry" so much as about temperature control. I'm more confident again now that I can keep my core at an adequate temp with this as the shell part of the solution (I have to admit that I don't get the whole fleece thing either, FWIW ... maybe it's a personal metabolism thing).
I reckon I might have some revivex or nikwax mailed to/from the trail a couple of times along the way to revive the DWR; that might help for a while, at any rate. If I find that my shell just isn't cutting it in the short term I'll make myself a rain smock out of a yard waste bag, and in the long term do a mail swap for my Packa.
In total, I'm saving something like 10 oz with this approach, given that with the Packa I'd be inclined to carry a windshirt, and with the OR Helium jacket not so much.
Even though I'm inclining away from your advice here, please understand that I seriously considered it, and very much appreciate you offering it.