Well, surprisingly no one has actually bothered to mention WHY the Saleen S7R makes less downforce than the street versions: ACO/FIA Mandated Flat Undertray.
The S7's undertray is classic GTP stuff with deep ground effect venturi tunnels, reminiscent actually of the XJ220. The major sports car sanctions of the world have long banned ground effects and mandate flat floors for all GT cars, thus the S7R doesn't have the added bonus of its production brother's tunnels.
Quite honestly, the S7 Twin Turbo is quite an impressive car, and my own stylist's eye says that it follows the classic styling maxim of "Longer, Lower, Wider" to a tee. Much more aesthetically pleasing than the bizzarely curvelinear Veyron.
Is the Veyron not as impressive as the Saleen? No. It to is ridiculously fast, and I do admire the unbridled decadence that the designers were allowed to build into the car (honestly, a diamond in the dashboard is just pure hedonism).
I'm a racer, and I probably wouldn't be spending 1.5million on a road car that wasn't a classic (1.5million would buy me a Bizzarini 5300GT Strada I'm sure), but given my own choice I'd choose the Saleen only because I prefer its styling, prefer its gearbox, and prefer its layout. Because I go to the track and campaign an ACTUAL RACE CAR, I'm not going to fret over a tenth of a second, performance wise, here or there compared to the car I don't like.