One more data point from this past weekend (my annual Presi-traverse-attempt-and-punt). Friday night it was about 5 degrees and blowing like hell, enough that spindrift was blowing under and into the vestibule of my tent where I was trying to use the stove. I set up the Reactor in my little bowl with a little bit of liquid water to get the canister warm-ish (water was actually very close to freezing and started to crystalize pretty quickly; I took the canister straight from my pack) and it lit up instantly. This was a canister that was about half full. I got snow melting right away, and the stove performed really well. It felt very predictable in the enclosed confines of my vestibule. No flare ups. I added a couple tablespoons of warm water from the pot to the bowl periodically to keep the canister warm. The speed at which it converted snow to boiling water was astonishing. For my tentmate and I, we went through about 8 oz of fuel total for Friday night and Saturday morning. That was for melting snow to boiling water for about 8 L of water total. The performance of the stove did not diminish - it was blazing hot right until the last sip of fuel was consumed and the stove went out. I switched to another canister and the stove was running again in less than 90 seconds.
From here on out, I plan to use the Reactor for pretty much all of my Winter trips. I'm sufficiently impressed. Maybe in the minus 20 range the Whisperlite will remain my go-to. I'll do that experiment in the future.