'locking up' in simple terms is the tyre road interface breaking down. Therefore as long as your brakes can cause the forces required to exceed the limits of grip available in this relationship then you can 'lock up' the wheels.All below assumes a well maintained fully operational setup.
If all other aspects remain the same, by changing the brake caliper system
(ignoring pad material and disk size) all your doing is changing the feel of the brakes, ie bite point, travel etc... The advantage of a bigger system is the way it deals with heat, ie being able to produce the same force after repeated braking by dissipating the heat better.So in my mind, unless you can cook your brakes while in intended use
(road/track) or the pedal feel does suit you, then probably you can leave the caliper alone. If you want to improve the car's resistance to lockup, look at better tyres.On converted car's the brakes will have to work harder due to the added mass.
But unless you exceed the weight of what toyota deemed the mk1 setup good enough for, ie a supercharger, t-bar with all the trimmings (1200+kgs?) then you should be ok for a road car. Obviously things have changed since toyota designed this system, with better tyre compounds and tyre designs. These will cause heat to build quicker in the braking system than intially designed, as will higher average speeds the car might be braking from.I think the reason anyone notices a differance is probably due to the bias changing.
I've driven a stock setup and a Front:st185, Rear:mk2 mr2 setup.
I believe the modified setup also has some master cylinder mod too (there was a purple anodized bit when I took it apart?). Both work fine, the mk1 setup is a good road setup, can lock wheels when required force is applied. And the modified setup is the same but can make many repeated stops from ***mph without fading but also has less travel and feels better under my foot. 
Sorry for essay.
