I've had a GPU that had similar specs to a UHD 620 before and I got around 20-25 FPS with Sonic Ether's shaders with a really low render distance. While that's technically "playable," that's really not a fun FPS to play at, regardless of how good Minecraft looks (given, it does look awesome). An Iris 655 will definitely be an improvement over the 620, but I don't know by how much. I would guess around an extra 20-25 FPS, so maybe a total of about 50?
Just out of curiosity: are you planning to buy some type of Macbook? Most Macbooks I've looked at have Iris GPUs. I would really recommend against buying one of those. You could buy a mid-level gaming laptop for half the price of a Macbook Pro, which will have far better performance for less money. The laptop I have only cost me $899 and runs Sonic Ether's at around 120 FPS with a medium render distance. Runs most other games at 60 FPS just fine as well.