...but in the corners, a car will usually has a significant cornering advantage due to having four contact patches
I agree with everything you said expect this:
4 contact patches have nothing to do with it. If you can imagine a bike with 4 wheels inline - it still wouldn't be able to roll through the corners as fast as a car.
I think it's WHERE the contact patches are placed that makes the difference. A mike must lean to turn and I think the factor that maxes out a bike's cornering speed is lean angle. A bike + rider hanging off as a total unit may produce a total lean angle of lets say 60 degrees. The footpegs are scraping, and the rider is hanging off so much that he is even tucking in his knee so that he can hang off farther. At this point, the balance of gravity and centrifugal force is at equilibrium and to gain more speed without increasing radius you must lean further which in this case - impossible.
Nomatter what tires you put on the bike, I think speed, radius, and lean angle would produce a certain amount of lateral G nomatter what bike you are on.
A Harley or even a bicycle at 30deg lean, at 40mph, in a 50 foot radius turn may produce .8 G's as an example. To increase speed, lean angle MUST increase. This equation is independant on tires. Your bike can be on a rail - literally - like a train and it would be the same.
Sports bikes compared to cruisers can lean farther but there is always the theoretical limit of 90 degrees lean if the bike had 0 width.
A car also has a theoretical limit but tires are always the weakest link before that theoretical limit.
Imagine a car with tires that produce unlimited traction. The limit would be where the centrifugal force will lift the inside tires off the ground and roll the car over.
I have never seen this happen unless the car hits a bump. Usually, the tires give out WAY before this limit is reached - nomatter how good your tires are. (Maybe a bus or Van is a different story).
In conclusion, without being an expert, I think the limiting factor of a bike's maximum lateral G's is not tires but the physical dimensions of the bike + rider that limit the lean angle possible.