Had a minute tonight and wanted something to do with GT2, so thanks for the question.
Same stock wheels (225/45R17), same stock suspension and ride height, same engine and powerband, same weight, horsepower and dimensions, same gear ratios, but different stock redline.
400R has an AT shift point of about 8150 RPMs, while the Preceiding model has a shift point of about 7650 RPMs. Notice the powerband for these cars peaks well below this, at 6500 RPMs, and drops quickly after that. So, if you're driving manual and are mindful of the powerband, these are the same cars (except the 400R gets the Race Mod and an extra 2 hp fully tuned.) But if you're driving automatic, the Preceiding model is actually faster, by a full half second over 1000m stock for example.
Edit: try these cars stock with just Medium Slicks at the Super Touring Car Trophy 591hp race at Midfield. If the RUF shows up, you probably have little chance of winning, but you can win against the Viper/Supra/Corvette. But it will be so close that if you're driving AT, you can see the difference between the 400R and the Preceiding model (which is maybe 1 second faster per lap.) If you can get through the final hairpin in first, be prepared to block the Viper down the straight to the finish... exciting race!
Edit 2: as a further aside, try these cars also, still with medium slicks, at the 2nd and 3rd GT Euro races at Grand Valley and Rome after you do stage 1 weight reduction. TVR's and RUF's give good competition. Then do stage 2 weight reduction and try the GT Pacific 2nd and 3rd events at Seattle and Laguna Seca for more great racing, this time against Vipers and NSX's. Extremely close racing, sometimes have to give a little love tap to get on the inside through the final corners to win.