Rear derailleur max capacity exceeded?

This relates to the lack of 'full groupsets' fitted to nearly every MTB sold today.

The spec of your bike is largely 'Deore XT M780', but you have an 'XTR M980' rear derailleur.

This is something seen on many many new bikes - a higher spec RD and lower spec elsewhere

The first and most important thing to note here is that XTR M980, XT M773, SLX M6663, and Deore M593 are the same generation (the first Shimano MTB 10-speed) and likely to be very similar.

While M980 is specced 41t, the RD-M773 introduced the same year was specced 43t, as indeed where all the other SGS 10-speed RDs except XTR.

If you look at the original XTR M980 spec, it was 44/32/22 plus 11-34 or 11-36 M980 cassette.

One observation I'd make here is that the 9-speed generation had 11-34 max cassette size, and 10-speed went to 11-36. XTR is always developed first, with the new 10-speed technology, and it's possible that the 11-36 cassette was a later addition, since it doesn't make any sense for the specs to say 41t capacity when the other specs add up to 43.

Note here that there are two M980 RDs, long-cage (GS) and super-long cage (SGS) (you have linked to M981, which is different though maybe not in important ways for this question)

According to the M980 tech doc, the inner plate for SGS is Y5XC10000

https://si.shimano.com/api/publish/storage/pdf/en/ev/RD-M980/EV-RD-M980-3085B.pdf

this was also used on the replacement model, M981. This replacement added direct mount for compatible framesets; it's just a removable b-link and doesn't affect the derailleur design

https://si.shimano.com/api/publish/storage/pdf/en/ev/RD-M981/EV-RD-M981-3429.pdf

According to this the exact same derailleur cage pattern is used on M781, which is specced for 43t.

Since M780/M781 and M980/M981 use the exact same cage (just with different materials), it follows that there is no difference in the cage capacity.

The use of different slant angles in the derailleur itself can result in a different capacity; to verify this definitively you'd need to compare an M98x series RD with a lower-series contemporary 10-speed RD.

However in my opinion it is vanishingly unlikely that they would use the same derailleur cage on XTR and XT of the same generation but change the derailleur to add 'extra capacity'.

The most reasonable conclusion is both derailleurs are exactly the same in function/capacity.

Since 'total capacity' is mostly a reflection of what Shimano sells, there's no particular reason to suspect it represents 'maximum capacity' - they are not going to test their RD with some random wide-range front crankset. So it might work perfectly, but Shimano don't care about such things and won't certify it. My guess is that since your RD is likely 43t capacity already, you would have no RD issue.

However your FD will work poorly since it has grooves in it for the middle ring, so unless you can find some FD designed for these gaps, then it won't work well. Of course if you change all three rings from 42/32/24, to 44/34/26, then I'd expect that to work well. But note that Shimano rings are matched - a 44t ring from a 44/34 triple is different from a 44t ring from a 44/32 triple - only the small ring is interchangeable - you'd also need to have a middle ring that matches to a given size lower for good shifting performance.

Looking at the existing 10-speed chainsets

https://docs.google.com/spreadsheets/d/1HdcNgmQKFA-lAqQJ3lXQ8SwA4tblAKiOXe9XYJJHYN8/edit#gid=375920416

the obvious switch from FC-M780 42/32/24 is FC-T781 44/32/24; i.e. the 44t chainring fromt that, and then FD-T780-6, which has the correct 12t top-middle jump, and 66-69 degree chainstay angle (FD-M780 is 10t top-middle)