Our experience has been that the load of the new, stiff belt puts a lot of stress on the old water pump bearings. Thus causing it to start to make noise, then if it breaks....bye bye motor. You would be silly to not put a new water pump on now as well as new rollers as well. If you buy the timing belt kit, you won't have a problem.
Plus even if you catch the water pump bearings, you still should put a new belt on anyways. Especially being a Subaru. We do not do timing belts on anything w/o doing a water pump and using a kit. EXCEPT for non-interference motors like Toyota's and older VW's when we know if something else fails nothing catastrophic will happen.
Your choice, other tech's welcome to chime in.