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forget the weight of 4 additional spokes per wheel
This will be negligible when youre using heavy shimano hubs. I can't think of a situation where 28H could ever be the best option. If you want a light/aero race wheelset you'll be using 20/24H, if youre commuting on training 28H will just be weaker with no noticeable benefits. Aero and weight between 28 and 32H is minimal but strength and stiffness suffers with 28H
Training/spare wheel's XT hubs are centre lock.
Thinking aloud:
I was hoping to drop the spoke count down to at least 28, of which the only Shimano option without a weird lacing would mean the CX75. Which are double the price of XT, weigh the same and have the same seals.
I suspect the best route would be to forget the weight of 4 additional spokes per wheel, stick to the XT hubs and use some lighter crabon rims.