A car with aftermarket stuff will allways be better and faster then a stock one, because tuners have optimized the car even more. According, to me, everything out of the box isnt worth anything. Tuned and tweaked cars will allways perform better, period. Even with the same HP, tuning can make a huge differance.
Simply NOT true. Many of the renowned tuners do not improve the car at all. Certainly added power will make a car faster, but that does not mean it will be quicker round a track or a twisty road. I've owned and modified plenty of cars over the years. My last M3 for example. It was a stock E46 M3 manual when I got it. After a while I bought a very expensive end allegedly superb coilover system from intrax. It was a none adjustable item and was supposedly tailor made for the car.
It didn't ruin it, but it made it a worse car in many ways. The comfort was gone and it was no quicker around the tracks I visit many times each year. Because of this I took it off and sold it with a massive loss. Then I bought StopTech discs, calipers and pads all round and some light weight OZ wheels with Toyo R888 tires. It made for a perfect car. The brakes is what made the car. Which is why it's a shame you can't change them in GT5. The tires weren't good for regular driving so I only used them on the track.
To claim that aftermarket parts make a car better is just plain stupid. Some do but most of it, the cheaper stuff, just ruins it and makes you seem like a fool. I love cars that are low and has all kinds of neat tuner parts on them. But I'm first and foremost a driving enthusiast not a 18 years old with a 10 year old Honda Civic. I look for balance and poise. I prefer handling and comfort before looks.
Most tuners and after market manufacturers are s**t. Some are very good, but most of them don't do enough research. This is why they all have the disclaimer about performance enhancement not guarranteed.
So to conclude I disagee strongly with your claim that after market parts ALWAYS makes a car better, because it is simply not true at all.
I'll add this example, I doubt you could make a E36 320i be as quick or as fast as a McLaren F1 just because you gave it more horsepower and wider tires than the McLaren. After all, there is a video on youtube where a Jun 1000bhp Supra has no chance of keeping up with a stock McLaren F1 in a straight line drag race, despite having better power to weight ratio and slicker tires.
Period!
I love this car too! 
