The standard micra brakes lock the wheels fine if the fluid, pads and rotors are healthy. Any healthy and decently setup brake system will lock the wheels without too much trouble.
The problem with the micra's original brakes comes from them not being able to take enough heat.
The reason we as micra drivers upgrade our brakes is to improve how they handle stressful braking situations (ie: slow down how fast they heat up, as rotors and friction surfaces won't work once outside their specified operating range).
Going to twin pot brakes gives an improvement in brake feel compared to one single large piston and more even braking/pad/rotor wear. Sliding calipers work fine on the street but in constantly high temp conditions they loose their effectiveness and the uneven pad and rotor wear causes issues. Not to mention they're made from strong cast iron, and twin pot calipers can be made from much lighter alloys due to the difference in forces they under when you step on the pedal and ram the little pistons out to push the pad onto the disc.
Those discs are very fat and I dare say complete overkill for most people BUT there's nothing 'wrong' with going overboard on your brakes (apart from increasing unsprung weight).
My view anyway.
