I asked on the lever ratio because of the turns! You should be able to see the ratio when you remove the pan. Depending on the ratio you have to adjust the front band. I don't think it will change the flare problem but it's always good to know the ratio and like that the band adjustments.

The biggest hole you can make is to remove the orifice as original! But then you will probably have to much overlap causing the 2-3 shift to feel hard and maybe noisy ("clonk" sound). Best to see this if you lift up the rear end in the air and start to accelerate and look at the wheels when the 2-3 shift comes. If they slow down a lot or even stop you got too much overlap.
I would guess you just need to increase the size of the orifice hole. Too much and it will start to have more overlap that makes you loose ET and also is hard on the transmission…