I feel the aero cross rails flex significantly on the OEM roof rack. I use a Yakima roll-on bike mount clamped to the OEM cross rails and my mountain and road bikes wave around when making turns and hitting bumps. Any of my kayaks cause a definite dip in the rail. I will resort to using the Thule adapter rail and my Thule accessories if I am taking the Impreza any distance with bikes or a kayak on the roof as the adapter sends the loads (forces) to the outer edges of the OEM cross rail.
The slightly thicker, stiffer rails on my wife's outback don't have any of these problems.
The Impreza rails are admittedly very quiet, and the flexing is not bad enough to make me spend the $$ to buy a complete Yakima or Thule replacement.