Low sprocket exceeds derailleur max teeth but total capacity within range of derailleur I plan on keeping a 2x10 drivetrain on a couple of bikes, and I've come across a sale on 10s cassettes. One of the cassettes is a Shimano HG-500 with 11-42t count. The front drive will be either 34-24 or 38-28. My rear derailleur is an XT RD-M781 SGS cage which specs out as max low sprocket of 36 and total capacity of 43t.  If I were to put the 11-42 cassette on, the total capacity count is 41.  I realize that Shimano is very conservative with their specs as far as maximum tooth counts, but six teeth over the max low sprocket spec may be a bit much.
Anyone care to chime in on any experiences with a similar set up? Or thoughts on the question, if I've exceeded the tooth count on the max low sprocket but not the total capacity of the rear derailleur, can I expect good shifting?
Notes: yes, I'd like the low ratio to defeat some steep berms that have thus far defeated me.
I typically match up a dedicated chain to a specific cassette, so the 11-42 would come with its own sized chain.
If the RD is suited for 36 teeth it may work with a 38 but highly unlikely with a 42. Shimano is conservative with teeth counts, you're right but still there are limits to get things to work. It is mainly a problem with geometry. The cage will have trouble to take up the extra length of chain. And also not to get into the way of the sprockets. You could try with one of these devices that put the RD 2cm lower but you'll trade it for less precise or more sluggish shifting.
The size of the front rings plays a lesser part if the difference the two is between more or less standard, although with smaller sized rings you may have trouble for FD height with a braze-on mount.