An STI is NOT a VERY HIGH PERFORMANCE machine btw... but to get back on topic:
Stability at 90-120 MPH range is defined not only by the weight, weight distribution including center of gravity, tires, and the suspension but also by the aerodynamic characteristics of the car.... you can put a big ass wing in the back, tweak the front spoiler and rear diffuser but the basic shape of the car will have a huge impact on the aerodynamics of the car.... a lot of people fail to realize how much work and effort is placed on the UNDERSIDE of a high performance car to increase stability at high speeds.
The reason that C43 AMG feels more stable at 145 is due to the sum of parts - its part weight, weight distribution, shape, suspension, aerodynamic characteristics, and tires - it is also far more expensive than a MS3 - 20 something grand is only going to get you so much.....
For the price range the MS3 holds its own on the stability side at high speed but the suspension is too soft, the rear too light, and the underside has very little in the aero design area for it to compete wth cars in the trully high performance arena.... the STI btw suffers from the same types of problems and what it gains in weight distribution it loses in the aerodynamics of a brick area.....(my def. of high performance starts at the likes of a C6 Corvette and the types of cars that can match it or beat it - sheer speed need not apply)
Look at the underside of a C6 corvette, porche, or SL55 AMG and you will notice that there is a lot of thought placed as to how air will flow under the car....they are designed to create a huge amount of downward force as speeds increase - it isnt all done by a big ass wing in the back
I was actually pleased with how the MS3 handles at high speed as I expected much worse but the car doesnt get too nervous. The only complaint is the suspension is too soft and noticeable at high speeds