You aren't going to like this answer but thats as good as those brakes are going to get.
First, keep in mind that plastic is made from oil. Your mags are plastic Oil, regardless of the end product, tends to have less than optimum coefficient of friction... this means they are slippery.
Second, U brakes are never meant to be able to stop on a dime (or even a dollar). They are intended to allow the rider to modulate their speed- to slow down at a controlled rate. Although Haro put sticky pads on the bike from the factory they aren't that great.
Third, a Gyro doesn't do you any benefit either. Despite vast improvements they still don't transmit braking forces well.
The only thing you can really do is replace the cables with stainless steel, the brake housings with compressionless type, buy the best Gyro split cable you can buy (they can be expensive!) and replace the brake pads with stickier ones.