As above, those brakes are the cheapest and most craptacular, although may still be acceptable. CHange the pads and you might see a significant difference. THe straddle cables are already nice and low so there is likely not much improvement to be made in setup.
As for changing to a lighter frame looking for braking improvements - no. A lighter frame will almost definitely have more flexible stays - all steels have approximately the same stiffness and density per unit of material and the only way to decrease weight is to use less material, which is possible with higher strength (ie hi ten vs cromoly) but will result in more flex. Perhaps an aluminum frame would have stiffer stays, but I don't think it's worth the trouble.
I would also be concerned that the bike is too small for you, but if you like riding it, to hell with what I think.