They all have one thing in common - the need to physically push a steel chain to one side or another. That requirement pretty much eliminates anything except steel as a cage material simply due to wear characteristics of the metal.
For the record, XTR front derailleurs have used aluminum cages in all the low-clamp models since the 950 series, with the sole exception of the 985 variant that's built for 2-ring XTR Race cranksets. I believe that holds true for e-type as well. Dura-Ace has also used aluminum FD cages since the 7700 series. Aluminum cages hold up OK, I think they're just expensive to make.
On the original topic... for the abovementioned reason, a low-clamp XTR FD would be the only Shimano mountain model that'll really give you a meaningful weight loss, and that's a lot of money, so the best bet is to stick with your Deore. Some weight-weenies will use a Dura-Ace front derailleur with a Speen pull-ratio adapter, which is considerably lighter yet.