For regular track use people are modding their cars for several reasons:
brakes are not the best for regular punishment and stock parts are way overpriced.
I assume from your avatar (maybe I have the track wrong) that you have tracked your car at QR. brake fade with the pedal mushing to the floor at the end of the back straight stopping from 250kmph to the hairpin of turn 3 is reason enough to want to upgrade, I'd rather not end up in the wall.
Stock tyres are ok but easily improved upon, again, way overpriced for regularly trashing on the circuit.
More power is so easy to obtain in this car, just as it is with your S15. why stay stock? it is actually quite cheap to add power to the R35.
I have found driving my car with the mods has become progressively more challenging as it is faster and capable of more. finding the limit is no easy task. sure I get fast times more easily than I would in a less capable car but this just raises the bar. shooting 57.5 on the sprint circuit with a mild Cobb tune on standard tyres was much easier to me than aiming for 54's with a bucket of mods. I am mentally shagged by the end of a session usually - maybe I'm soft
I dont think anyone would disagree with you on improving their skill levels. whilst people post about their car mods openly, not many post about their driver training. Just because it isnt posted doesnt mean it isnt happening. most of the people you read that are modding their car for track work are serious about improving their skills. many take on instruction/practice sessions, read literature on racing and car setup extensively (you should see my library), talk widely with professionals and undertake complex data analysis. no one I know thrashes it blindly on the track and then modifies the car without thought of their own ability and how to improve.