GT4's Supercar Festival... it was a way for GT gamers to get their love of supercars and high-hp exotics and share it against others. Yes, this was a championship for you all to use supercars in high-stakes action. Sure, the supercars weren't the ones all the speedy types wanted, but at least you used these automobiles in safe, sane racing environments. So if you're used to this in the PGR series, you'll just need to adjust your driving style a bit for GT4.
Something I've kind of taught myself was to try not to modify the horsepower. When I got stuff like horsepower add-ons, it only upset the balance of the cars. Even considering that you MUST use Standard or Sports tires, it just makes things worse. Racing tires help make the balance of a more powerful car better. Since you can't use Racing tires of any kind, you must go with Standard or Sports Tires. Don't forget that your car must have 493hp or more to compete.
People talk about the Ford GT being bad to race with. I think I used a Ford GT at one point. I've also used a Callaway C12, a car I wanted to use as my little race car. I've won and lost races, but I figure that adding horsepower only means extra tuning. So I kept things simple after thinking about it. So if your car has more than 493hp and you have enough finances for tuning, just drop the weight and look for modifications which don't add horsepower. The only exceptions to this are maybe the mufflers and anything besides the NA, Turbo, or Supercharging options. I haven't played the game in a while, so I haven't tested this myself.
If you want a supercar with more than enough horsepower and can be a contender, the Ford GT can be quite a "bargain" at 160K Cr. The Saleen S7 isn't a bad choice either, but much more expensive at about 420K Cr. The 2003 Dodge Viper is torque-happy, but carries some wicked horsepower. My final recommendation is the only non-American one- the Mercedes-Benz SLR McLaren. It has the horsepower, it has the performance, but it's the most expensive at maybe 100K less than most rally cars in GT4. After all, this is the SUPERCAR Festival. Supercars mean supercar money. So if you don't have the paper, don't hit the series until you get the money flowing, you dig me? Alright... make it happen!