Colin, I can second the Si brakes. My friend's Si will lock up the back tire no problemo.
As for getting better stopping power on the puch brakes... try this... hopefully I can explain it well.
Take the brake shoes completely off of the brake plate. On the little pivoting piece that actually turns and presses the brake shoes outward, wrap a strip of steel around it. The steel strip should be the width of the pivoting thing and just long enough to meet end to end when wrapped around one time.
Think of it as wrapping a stick of gum around it until the ends meet.
This extra material between the pivoting piece and the brake shoes causes the shoes to stick out further and with more force when you squeeze the brake lever - and you'll stop harder.
Kinda tricky to explain, but I've done on several bikes and it seems to help quite a bit. Especially if the brake pads are a bit on the thin/worn side...