Fantastic boot with great flexibility in walk mode, but stiff enough to enjoy the downhill. Will need to get a gaiter for longer trips in very cold weather.
For my first use with the S/Lab X-Alp, I took them up the north face of North Maroon and the day after up South Maroon with a descent of the East Face to the Bell Chord. These boots were amazing with skinning, bootpacking, scrambling, and skiing down. At no point did I feel these boots would compromise my speed on the uphill, control on the climbs, and capacity to descend with critical jump turns.
What a stellar couple days to break in your new S/Lab X-Alp boots! Glad to hear you're impressed with the boots. I have a set myself and share similar sentiments as you regarding their uphill and downhill performance - they are definitely an amazing tool for light + fast touring and ski mountaineering missions.