I am pretty sure it is a somewhat reputed conjecture, but I do not have a clear reference where it is stated. 

**Edit:** It is stated in Gromov's "Metric structures for Riemannian and non-Riemannian spaces", 6.28 (1/2+) (in fact, it is stated for all rank one symmetric spaces).

It might be evoked in a paper of Hsiang and Hsiang in Inventiones, where they prove that the isoperimetric domains in products of hyperbolic and euclidean spaces are invariant under the group of all isometries fixing the center of gravity. It seems a reasonable conjecture that this is true in all symmetric spaces of non-positive curvature. That conjecture might be stated in the Hsiang and Hsiang paper, and is a broad generalization of the conjecture you are interested in.