I have exactly the same xenia, that I got from my LFS. It's been in the tank for a couple of months, and hasn't done too much, other than growing new fronds. I think it's going to split, but it's starting from the top of the trunk section. I have exactly the same lighting, and it's in a 20g tank.
I'd love to see some pics if you have any. I really like the Xenias as they are a unique coral (just like all the other corals) *lol*.
As was told to me by Sally Jo at GARF, you will need to prune or frag the branches (arms) when they get bigger to keep the Xenia thriving. I was able to cut an arm off mine and it is growing very well in my 7 gallon prop tank.
Cheers,
Alex