According to Lindarets the Goatlink is nearly as good as the Rad cage with a 40, the gap widens when you go to 42t. They reckon it’s also more of a budget thing, if you want to go the whole hog with XTR he says the Goatlink isn’t as good, if you just want to try and improve things with an SLX mech it’s worth using the goat.
I’ve just fitted a 40t to my xt cassette/ Zee mech and without the Goatlink I would have needed a few turns of the B screw to get clearance, I installed it and it’s back to where I started. I had to tighten the cable a a few clicks but shifting is good except down the bottom of the cassette where the 15t was removed.
My other bike I’ll convert to One Up 42t with a Radr cage.