Pull repeatedly on the levers and look for movement in cable housing, hanger, etc. It all adds up and is the bane of centerpulls, which already have plenty of flex in the caliper itself. Sharp bends in the housing introduce friction that also saps applied force. Better cable inner wires are pulled through a die to flatten the outside surface and reduce friction. The biggest improvement, though, will be in switching to aero brake levers. Providing you don't have so much flex that you run out of cable pull, their increased mechanical advantage will make all the difference. If you still require more stopping power, a suitable length dual-pivot caliper is probably your best bet. I've seen some rims on which almost nothing seems to work well. I always figured that there was something about the type of aluminum, just as there are some rims that are much more likely to give up aluminum that embeds into the pads, the Weinmann concave being the first that comes to mind. There can also be a huge difference in the friction provided by new pads and that of old, dried up pads that otherwise look fine.