From what I understand the smaller ones are more effecient on the big end period. but I believe they go with the bigger in diameter because the 8" can only handle so much power. Most of the outlaw converters I see are 9.5".. Esp when you get into the big blocks with all that torque.JMO
Here is some info from Neal Chance Converters..
http://www.racingconverters.com/converters.html