I'm not sure what game you are playing. A premium car versus a standard car is only relevant to the driver and observers at the highest LOD. I don't know where you are getting your numbers from. I've seen 16 premium cars on track online many times in GT6(In fact, I see it almost weekly in league racing), so your limit of 8 makes no sense.
I'm not seeing reduced car count, beyond the topic of this thread (more people playing GT5 online). Also, there is a big difference between offline and online physics in GT5. I've had my car slammed back to the ground in GT5 when there should have been a roll over. In GT6, the difference is far less noticeable (in fact, I haven't noticed a difference at all, if there is one). That right there will affect performance. Also, more tracks are using the time of day rendering which forces dynamic shadows and, in many cases, dynamic lights from the cars. Not only is that costly from a pure rendering standpoint, but networking that is more costly.
I turned off my anti-aliasing (sharpen image option) and I've found my frame rate is much better than the default.
Under the same conditions as GT5, it performs better. In GT5, I refused to race on the weather enabled tracks online because of the terrible frame rate and screen tear. Now, on those tracks in GT6, with the anti-aliasing off, performance is far better. The frame rate is acceptable all the way up to 16 cars and no screen tear.