RB is right on this one. Puting a rotor in the center of the kart does affect a lot of things, including the flex. Many tuners use stiffeners in the center as a way to move the flex to the outside of the bearings.
The bearing acts as a fulcrum point. As the axle flexes upwards under cornering, the center part flexes downward. This is how our Pro axle works, thicker in the center, thinner outboard of the bearings.
But there is another problem with putting a rotor in the center. It too involves axle flex but what happens is the flex distorts the rotating plane in which the rotor spins. Yet the caliper is fixed. So what will happen is the rotor will rub on one of the brake pads creating drag. That is the main reason the caliper is moved so close to the cassette. The farther you mount the rotor away from the fulcrum point, (bearing), the farther you have to retract the pads to prevent rubbing. This greatly increases the pedal travel you need.
Mike wants to know about the heat sink. I’m not sure how valuable they are. I’ve used them, they do seem to reduce heat. But a better solution maybe to just better maintain the brakes.
I found out some interesting info in the past week. Dot 4 and 5 oils do have higher boiling points that Dot 3. However, they also expand more at a given temp than Dot 3. We also use steel brake lines that do not expand when pressure is applied. This creates problems in kart brakes.
If you fill the resevouir to the top, there is no place for the hot oil to expand in to and it causes the brakes to drag. Also, most Euro brakes are not designed to use silicone based fluids and that causes the seals to expand, creating more drag. The best solution is to use a quality Dot 3 and change it out regularly.