The “big ring to big ring plus two” rule would suggest I don’t need to change anything, but I’d rather not end up tearing my hanger off – what say you lot?

It should be fine, assuming your not changing any other variables

The required chain length is independent to the derailleur cage size.

Mechs have 2 vital statistics: max sprocket size (usually MTB 36T, Road 28T) and range (slack take-up that is cage dependent)

You get the chain length right and can fit any appropriate type mech….if the cage is too short, you end up with the chain hanging loose.

Very loose guide is 1 chainring for Short, 2 chainrings for Mid and 3 chainrings for Long Cage.

PaulD

What is wrong with the short cage mech? If that was doing the job then stay with short cage…improved clearance, ‘quicker’ changes (assuming you time things in nanoseconds!), lighter weight, looks more Ggggrrrr.

It should be fine but obviously won’t get the benefit of the longer mech (which would work with a bit more chain and therefore allow ‘easier’ big/big gear selections).

If the short cage is doing the job then stick with that length.

Going from Sram X9 SS to a Shadow Plus XTR, can’t get the XTR in short cage, and one or two clearance probs with the X9.