All great information.
My trip this year isn't actually the JMT (but I don't think I'm too off topic). With my father and brother, I'm doing about 50 miles from Agnew Meadows to Yosemite Valley, hoping to do a fair bit off trail wandering between Marie or Davis Lakes and the Upper Lyell Fork of the Merced. So I won't be scrambling up Whitney, but I'll be up high a fair bit.
I don't have the miles or nights that lots of guys do, which is why I'm happy to absorb more than contribute around here, but I'd say I'm generally confident with tents, and I'm cautiously optimistic with tarps. I'm used to shoulder season temps occasionally dipping down to 20 up here in the PNW, and I've tarped in the cold/wet up here successfully by myself. And the more I've continued to learn about the high sierra in September, the more our clothing list has become what I usually take during shoulder season in Washington. I'm confident we've got the right insulation for 20 degree nights, if we're dry.
Which leads to the part where I lack confidence with tarps: I've never tried to keep 3 people dry with a 10x10 tarp in strong wind, at 10,000 feet, in potentially much more exposed locations. I pitch the tarp with trekking poles, and have done so successfully in what I'd call breezy weather, but nothing like 30MPH winds. And as Bob pointed out, wet gear can be hairy situation.
At this point, I'm leaning toward bringing a tent and a tarp. I don't have the money for a 3-person pyramid. My Fly Creek UL3 is ~3lbs. Adding a ~2lb tarp, I'd be looking at 5lbs shelter. I think I'm fine with that. I'd be able to test run the tarp with a fallback option. I'd much rather shoulder the extra weight than make for a miserable trip.