Great care needs to be taken when fitting Metro Hubs to a Mini or even using the sealed Metro Joints in KADs and a few other after market hubs that used them. The stock Mini Ball Joint has 500 of swivel movement and they need every degree of that. While the bump and rebound stops are in place and in good order, you can possibly get away with a little less, but it's not advised as bump and rebound stops seldom are examined properly, they can fail with little warning, if any, more so these days with the highly variable quality of the after market parts that is all that's available to us.
I know the Quinton Hazel MKI Metro Ball Joints do have enough swivel angle, however, there's been a few other brands I've checked that don't and fall well short. This too is where care is needed with after market hubs as while your choice in ball joints might have enough swivel angle, the angle to which they may fit in to after market hubs may not be the same as the factory had them set to. The issue here being that on full Droop or full Compression of the Suspension the pin can hit the ball joint outer cup before these movements are reached, after which, instead of the joint moving, it will try to be bent, being on the harder side, this leads to them in short order breaking.
Getting a little off topic here, when ever any Negative Camber is dialed in, it is very important to pack up the Rebound Stop. Without doing so, on full droop, this situation of running out of ball joint swivel angle occurs on the lower ball joint, leading to breakage if them.
The factory, while not giving the back ground as to why, did advise of the need to do this;-