Im assuming that by giant cup you are meaning Amplexidiscus fenestrafer rather than the larger rhodactus hairy mushrooms.
Mine has done well high under 175w metal halides, and also lower, though it has bleached on one side since I moved it lower because that side was too shaded. Ive moved it again to give the bleached area more light so I will see how it comes back.
A fts from prior to moving it last September, It was opening 16in+ at this time.
I moved it to the lower right corner, and it wrapped around the rock along the right side glass, so the half that was going back along the rock was not getting enough light.
At the size mine is at I wouldnt consider attempting to split it directly. When I moved mine it was attached to multiple rocks and I accidently tore the foot in a couple spots, leaving behind small spots of flesh on the rocks it was removed from. Those small fleshy bits actually healed over and I now have at least 2 1-1.5in frags growing. The moving process was highly stressful to the large mushroom, it produced large quantities of slime immediately, to the point I was afraid it would effect the other tank inhabitants, then for several weeks afterward it didnt open well, and the mouth was gaping. At this point, 6months later, Im fairly confident that the origional mushroom is healthy again.
The white patches to the left of the yuma in this picture are the 2 accidental frags I had that survived from foot tissue that tore off.
