The rules are there to make it so the cars will have sinilar specs, however the cars themselves are not almost the same. Thwe power outputs should be similar and the straight line speed should be similar, but each car is still different in the size of the chassis, CoG, wheelbase, engine position, suspension, and the cars settings as well as the driver. So you can have car A and car B both having a 4.6 litre V8 pushing 600Bhp, car A has a lower CoG and a wider track and also has a slightly longer wheelbase than car B, they are racing at a high speed circuit like Enna. The only way car B will beat car A is if the driver of car A is bad or crashes, car A retuires from the race, or the mechanics set up car A all wrongfor the track. Basically, car A is the better of the two and if the setups correct for the car, it will be noticably so.
This i a generalisation ofcourse, there may be exeptions but whilst the power figures and certain components will be similar or the same from car to car, theres still a lot of parts that don't effect power figures ect that may be different. Some race series require almost identical cars, or even identical cars so it's only the setup, pit strategy and the driver that decides who wins but GT racing gives you an element of freedom in certain areas of the cars design.
And good luck at UTI, do you start this year then?