From what I have read most people who use to get the larger flapper were using that to get rid of boost creep when running large exhaust. Back then 20 psi was considered a lot and so trying to keep the tubro below that was hard without good porting and a large flapper.
Today most people are running a lot more boost on that turbo, say 25+, so creep usually isn't an issue but the larger flapper area is. With more area the pressure required to keep the flapper closed is much greater so running the smaller flapper is better for high boost situations.
My .02