Many people tend to go too big on turbos.
You are considering 6mm elements so no need for a bigger turbo unless you like to drive a dyno queen with huge lag if your ip can spool the turbo at all.
Start with the ip 6.5 or bigger and leave all the other things stock for the moment.
As written I ran the 8.5 mm ip quite a while to see what is going on with performance and EGT then 3" exhaust (header still stock) and still with the stock turbo.
Go step by step and start with bigger elements.

Tom