Is it worthwhile?
No. I spent a fair amount of time with the dremel and the drill carving away large chunks of metal and the net result was a 15g weight reduction from 240g stock to 225g after tuning.
A shadow XTR rear derailleur weighs about 180g out of the box and there is probably no amount of tuning that will get a normal XT rear derailleur down to that weight.
Tuning potential
The body
The body of the derailleur has a lot of unnecessary aluminum that can be removed with a dremel or die grinder if you are careful and patient.
Here's a view of the top knuckle with lots of meat removed:
There is also lots of excess metal in the bottom part of the derailleur:
Jockey wheels
The stock jockey wheels on the XT rear derailleur weigh 10g each. One features a sealed bearing and the other a ceramic hybrid bearing. The derailleur that I tuned already had about 40,000km on it and the bearings in both jockey wheels were still good.
Most trick looking aftermartket jockey wheels are in the 8-10g weight range so there is not much weight-loss potential in the jockey wheels. Indeed, to save a bit of weight on the standard jockey wheels you could drill them as well.
Pivot Bolt
The standard pivot bolt weighs 14g. An aluminium version from Toronto Cycles is 3.5g. For a stronger titanium version its 12g.
Jockey wheel bolts
These bolts are really small, so pretty minor weight savings by replacing with aluminium. Maybe 1g total.
Problems with these derailleurs
The weak point with this derailleur has been the pivots which over time wear so that the derailleur develops 'slop' which results in less precise shifting. The pivot pins are not replaceable as they are crimped into the body. I am not sure if they run on bushings, but based on the amount of slop that they have developed I suspect that there are no bushings and they just run straight into the base aluminium.
Aluminium bolt tuning
By replacing the pivot bolt, both jockey wheel bolts, the barrel adjuster and b-tension screw with aluminium bolts and the cable clamp screw with a titanium button head allen bolt resulted in a saving of 13g.
Final weight after tuning - 212g.