What do you mean with "too tense" ?
I would think the relevant criteria to decide if the backlines are "too short" or if something is wrong would be if the kite back-stalls or not or if the trim/depower doesn't provide the expected wind range and keeps it powered up all the way all of the time.
It might just be that the kite has more bar pressure then you are experienced to?
FYI: You can change the bridle connection points to change the bar pressure a bit although its not a gigantic difference.
At least with no slack on the back lines you get a more responsive kite...
I do remember having some slack on my XRs in extreme conditions on my 8M kite when there is some trim applied but usually I rode my larger sized XR kites without any trim.
Has been a while since I touched my XRs though since I am mostly on a foil these days.