Hi Ken, the only difference between the carbs is the venturi progression hole size at the bottom of the venturi.
I think all new engines come with 8.5 now, but you can easily change the venturi to switch a carb from 8.5 to 12.5 or vicky verca.
The progression hole is only effective just as you open the throttle so theoretically you will get a leaner and hopefully cleaner pickup out of the corner.
It wouldn't make much difference if the track is twisty or long because the venturi throat size is identical.
You are better off altering the float heights on the 8.5 but you could probably get a 12.5 to work in exactly the same way as 8.5 by changing the float heights and it may in fact be better.
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