Need for Speed NFS Pro Street Tips, Help, Tricks, Hints Cheat Codes and Strategy

I really need help ....
The trouble is, I see and hear many people have cars that are around 1000 BHP mark. I have tuned, bought the best parts (stage 4, also including the best bodykit) for many of my cars, and still I cannot get the BHP even close to 1000 BHP. One example is my Ford GT500. It's BHP is at 683 (this was the best BHP that came up when I tuned it)
There are many posts on youtube showing their GT500's with about 900 BHP!!!!
This is for PlayStaion3
Is there anything im doing wrong?
Please help ......