I'm going to go with the train of thought that a 10" spring will give more travel. Good on dirt, maybe not as necessary on pavement.
As I've always been a pavement guy, travel is usually limited to 1/2" - 3/4" ( a bit more on right rear). Sure, you can get more travel; But that is usually due to weak spring rate or very aggressive driving style. To each his/her own. For now, I will take go with the taller 10" spring.
Running in auto cross with the dwarf car will be very interesting. Shock travel and spring rates will be very interesting to watch. My SCCA experience has been limited to production class. So I didn't have the option to run different suspension set ups. My modified experiences have have taught me about bottoming out the suspension. It's going to be different for everyone. Circle track differs from road course. I'll just have to wait and see where where I end up. I hope I have my math right! Only time will tell.
Maybe CJ can give us some thoughts on the subject. ( I know; I'm putting you on the spot.
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Steve