Correct me if I'm wrong but this is what I have discovered thus far regarding shuffle racing.
Here is my mathematical theorem:
Shuffle ratio (times) difference in overall time with Race leader (divided) by the position you've managed to place at the end of race (equals) the car you will receive in the next race.
Example of my formula:
(Shuffle ratio) * (Race leader overall time difference) / (Final pole position) = amount of Next higher HP car
1 * 32.56 seconds / 8th place = Higher HP car
So basically, in order to obtain the highest HP vehicle in the next race; you must have the shuffle ratio to the max and finish in last place with the worst time possible.
Although, I believe there are only a select few of cars available to be driven while in shuffle mode; which means all the cars in shuffle mode will always be the same preset cars during random slotting/lottery (06 RSX, GTO, NSX etc..).
