That's awesome you have a light Instinct -- I think that bike in its current manifestation makes a terrific light trail bike. Most people end up on an Instinct out of a desire to build it as a rowdier all-mountain rig but it has great capabilities as a more pedal-happy platform as well. When I first built mine it was a bit under 28lbs at 140/140 with a Fox 34 and lighter wheels and tires, but it was still a capable bike in chunky desert terrain (awesome in a place like St George). Mind you, I later went the BC edition route and the bike gained 3.5lbs (and some descending chops, especially with the DPX2), but I think the first iteration was pretty neat.
I ended up building up a Ripley v4 to fill the slot between my Instinct BC and my XC race bike, and, suffice to say, didn't love the bike. It was certainly a lively pedaler and I didn't mind climbing on it, but it felt waaay too much like an enduro bike in its geometry (esp. seat tube angle and looong wheel base) without the suspension plushness or traction to back up the geo. The DW-Link tuning on the current Ibis bikes are, in my opinion, amazing for smooth and pumpy terrain, and absolutely unpredictable in slow-speed chunder; nowhere is this more clear than with the shock tune on the DPS for the Ripley. Very harsh, spikey and lots of rear wheel skittering. (Many folks end up changing out the DPS for something different; I briefly tried a Super Deluxe but didn't find it changed the performance enough from stock.)
If you are located somewhere like the Wasatch Front, northern California or the CO Front Range, where there are lots of long and steep-ish climbs on fire roads or switchbacking climbing trails, the Ripley will definitely out-climb the Instinct -- not by a mile, but by a noticeable margin. The steep seat angle will let you grind up huge climbs with a minimum of hamstring/back fatigue and you won't have to get in/out of the saddle much at all. If you can get the rear end feeling OK, you may find the Ripley kills it on those big-climbs-to-big-descents, "winch and plummet" types of rides. Especially since you can build it at least as light, if not lighter than the Instinct. BUT, any time the riding gets technical, the Ripley will buck you around, and won't have nearly the traction (up or down) as the Instinct.
While neither of them can be built particularly light, I think the Revel Rascal and the Pivot Switchblade are two bikes you might want to consider. I'm a bigger guy at 190lbs ready to ride and the lateral stiffness of either bike is quite noticeable coming from an Instinct -- much more precise steering and less rear-end wander in rock gardens. They're both phenomenal climbers, with the Rascal seeming to excel more in slower-speed, ratchet-pedaling jumbly tech and the Switchblade more on "power moves" and quick bursts. I don't quite need Switchblade-level travel (the 155/160 BC edition was also too much bike for me, except on days of lift-served riding), so I went Rascal -- which has been a phenomenal bike for the more backwoods, rough-cut New England trails I've mostly rode during Covid. Riding a CBF bike is, it seems to me, well worth the weight penalty -- my XL Rascal is about 600g more than my XL Instinct frame was. (Cable routing is notably better, though!)
I haven't rode the 2021 Stumpjumper but if I was dead-set on building a sub-27lb trail bike that would probably be the direction I would go. I didn't much care for the kinematics of the older Stumpy but I'm intrigued by the changes and the flex pivot on the new one.
