I've same model, +200 flights with Wren 80, no problems at all and fast enough and any acro is possible. Tarmac rwy in my case but I'm sure no probs from the grass too with an 80N and maybe with 60N too if you have enough space of course.
I'm also pretty sure no problems with a 60N, maybe slighly longer take off roll but after that it will fly beautifully. In flight Xcalibur if very clean and efficient, to be a twin tal boom jet. I also think to remember that when Dave Wilshere started to fly and test it few years ago, he had the chance to fly it with a 60N with no problems.
You will have a nice flying Xcalibur and lot of fun.
Enjoy the building and the model!
Denis