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I'll nerd-bite.. A 29er wheel has about 7% more OD than 650B, more stuff at the edge, it'll weigh about 7% more. The only thing that matters is the weight since the bigger wheel rotates slower at a given speed. 7% isn't enough for me to care tbh as wheels affect more important things when it comes to off-road riding.
My take on it seems to be about bump frequency and amplitude. Frequency is handled by volume, amplitude is handled by OD and roll-over. 650B is great for a road-gravel bike as it's got the volume potential at a lower weight, works well on fast, small bumps. 29ers can handle both so they make more sense for an XC MTB.
I like 650 x 50 for road and light gravel, basically anything where drop bars make sense. Another benefit to 650B as Ed said is you can get guards in and still have a fairly agile road geometry.
When the bumps get bigger and chunkier I'd go 29" but I'd also drop the drops by then, it's XC MTB time and I'd rather use an Jones bar or some sort of MTB bar than drops. With the wheel capability comes the want to handle the bike differently, for me anyway.A 29" drop-bar like the Cutthroat could be good if you have a lot of that middle ground riding, neither road/gravel nor XC MTB. Most of my riding is more one or the other or I prefer a bike that has clear advantages and disadvantages vs something in that middle ground.
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jameso
Netakure
In the hope of getting someone else to chime in, I think your estimate is correct for the tyre, but ignores the difference in wheel (bigger rim, bigger spokes) and rotor (as ed said, you need bigger rotors for 29ers)
If you take fortus 23s, there's about 100g for two wheels so...I guess... including rotors...maybe 200...250 grammes for a pair of 27.5 vs 29ers.
Which doesn't seem like much to me. Seems like 29ers are the way forward.