The problem will be escorting these ships... it is not exactly feasible to have the ships you are escorting at cruise be able to outrun your escorts when they are at full power. You will need to setup a chain of escorts to escort such a ship to its destination. It works when there is short distances, but across the ocean, it is impossible.
Also, some catamaran designs are not suited for open ocean use... especially very rough sea conditions, where only monohulled ships and SWATH ships will have good seakeeping. And SWATH ships have the disadvantage that they are very expensive, have a complex control system, a deep draught, high maintenance, a high power requirement (will require 80% more power than a equivalent catamaran), and limited speed. So you will have soldiers that are soon puking and very sick after an hour long voyage. Not good for morale and warfighting. Also, stored equipment can become loose, and bang into each other. In the end, in a moderately rough seas, you get banged up and dented equipment to land.
Furthermore, a typical fast RO-RO carrier will have approximately half the capacity of an average conventional RO-RO carrier at almost twice the speed. If you need speed, send it by air. If you need to take lots of it, send it by ship.