"Safety" extension levers are a bad idea. The aluminum they are made of is too flexible for adequate leverage unless the brake pads are perfectly adjusted and not the slightest bit worn. You're really better off getting a new set of Aero style levers, with an additional set of interrupter levers for the flats of the bars if desired, for real braking power.

Dual pivot caliper brakes as you've suggested will also help immensely. They have significantly better mechanical advantage of single pivot calipers.