Not certain on the weight it adds, but I have been very happy with my 11 spd XT setup.
Picked up a new XT build kit for a good price, the rear D was a direct mount so I needed to pick up the knuckle to connect to hanger, found someone locally that was selling a 11 spd Goat Link, so that got added on.
94 BCD XT cranks (175mm), with 32t cog, using an 11-42 cassette, picked up the OneUp 50t Shark kit (50t cog, with cage arms and 18t cog (replacing 17t + 19t). Yes, you have to take your rear d apart to change the cage arms, but it is pretty easy to do.
Shifting into 50 takes a little longer than other shifts, but is still good and smooth. I am climbing a lot of stuff in the 2nd and 3rd gear (42t & 37T) with the 32 front and when I need to rest and spin the 50t is a great bail-out.
Of course it could be lighter, but the simplicity of the system, no dropped chains, super quiet and reasonably cheap is worth the extra bit of weight as compared to the SRAM Eagle 12 speed cost.
I mention the OneUp stuff as I believe the big difference on simply going to a 11-50 cassette with a stock Shimano 11 spd rear D is that the amount of tension on derailleur by cranking in the b-tension screw puts a lot of load on the derailleur. The One up cage arms, and perhaps the goat link, allow the 50t cog to be used with pretty much normal b-tension settings.