I believe that because of the way SSD works, 8 cars is not possible. Even one additional car makes the encoded data stream too long for reliable and responsive operation of all cars.
On the bright side, if more than 6 cars is important to you, but you've already got a heavy investment in SSD, oXigen will be an easy and relatively painless upgrade path, allowing up to 20 cars on as many lanes as you care to cram in. oXigen and a dozen cars would make 4 lanes a lot more fun, as I agree that 4 is too many for 6 cars, even for only part of the lap.
The tracks of the digital club I'm racing with are all 2 lanes, even after some experimentation with 4 lanes. It only takes two cars to crash, so it's a lot more about experienced drivers that makes the difference for 6 cars on 2 lanes fun, or carnage.