The LS55R engine in the C7R is an LS7 derived motor with direct injection, limited to 5.5l by the class regulations. They were able to add direct injection when the LT1 made it into production. With the current limitations and restrictors, the engine makes around 550hp. Unrestricted, however, it's around 750hp.
You really can't -- and shouldn't -- compare motors across the GTLM board by looking at vehicle speed. Those cars are vastly different, and IMSA/NASCAR is always trying to even it out by limiting power, weight, downforce, fuel tank size, and so on. Comparing the race cars to each other, and trying to apply the same difference to the street cars, just becomes an exercise in futility.
Moving on, the GT gets so shitty gas mileage because of the antilag constantly running. They're pumping gas through it to keep the turbos spooled. It's no different than the current M3/M4 in Sport Plus mode. The GT does have some nice tech. I'm particularly curious about the suspension and aero adjustabilities being run off the power steering pump.
...on a more personal level; plenty of other cars out there I'd rather spend half a million on.
You can't just stick a DOHC engine in the C7R. No production Corvette has that, so they're not allowed to use it.
Yes, Porsche was stretching it with the new, mid-engined 911RSR.