Over 9000 max just by drifting the festival circle in my Ruf CTR Yellowbird. A continued drift about 4 times around nets you 9,000 or more. You just have to watch out for the dumbass festival goers who will crash into you.
I maxed my over 9k by drifting in an S15 with the R34 swap, and Interstate high speed runs... drifting over 9k is easy seeing as a monkey can drift now... And I've found just by drifting through red rocks with a little bit of effort I can get 125k points every time, almost never any traffic up there.
In Horizon?? Solution = Silvia S15 with I6TT R34 swap max upgrades street compound tires front and rear full width That is probably one of the most forgiving drift cars in Forza especially Horizon.