The XHP50.2 is fairly safe to slice the dome as its a flip chip LED, The bonding wires are hidden. I used a 1mm Thick washer on mine and it turned out okay. You can probably go 0.80mm you would be able to see if its to low from eyeing it. Just put the washer around the LED, So the LED sits in the middle hole of the washer.
I killed a SST-40 slicing it when i hit the bonding wires. These are defiantly easier to slice, Just make sure the dome is flat after slicing and has no odd bit sticking out. The washer helps make it flat.