Replace V-Brakes with Caliper Brakes on a Flat Bar Road bike
Generally, V brakes have much better stopping power than calipers. And I'm guessing that your frame will require calipers with a pretty long reach, which makes it even worse. Forget about caliper brakes - concentrate on making your existing brakes work better. 

First thing you should do is to check if anything's malfunctioning: are the cables sliding freely? Are any of the moving parts binding? Are the pads contacting the rim squarely? Next step is to upgrade the brake pads.

If you do switch, then yes, you'll need new levers. Caliper and cantilever brakes: short pull, V brakes: long pull.