If just the tank is losing pressure then it could be one of the tank fittings or it could be the compressor check valve.
To test the check valve, first remove the gray filter from the front of the compressor. Then turn the compressor on, with the compressor running the air filter port should...
What Sharad said, the 550 actually will go lower thanks to the IRS, look at the rear tire tuck compared to a bagged S197. Although it may not look like it goes as low because of the body work. I think we will see some pretty crazy builds once body kits are available for these cars!