The rational for using concentrated fix for RC prints is rather meaningless as the support does not suck up fixer, and the emulsion is fixed when it is fixed and thus can wash out in short or long times depending on concentration.

HOWEVER, the argument fails when we get to FB paper. There is an affinity between the fibers and the Baryta subbing for Hypo and Silver Hypo salts and this means that you can require much longer times for FB paper if you use concentrated fixer. I suggest that in this case, you follow the manufacturers recommendations.