seemingly u need an elbo just a little bit bigger than the blades and wastegate for better spooling. im goin to make 1 the size he thinks and a 3 inch and put both on the rollers to see wots best proof will be in the pudin.. he recons u shouldnt belive every thing u read cos every body recons bigger the beter as i said goin to give it a try as wont cost much
whe you do a 3" test are you going to fit it to a 3" system? otherwise your going to have a 0.5" step. i havent read that in magazines, read it in various books. also, 3" pipe can be make to fit the exhaust flange perfect without any cutting or welding, a 2.5" you will need to cut and weld extra pipe to as the circumference is smaller than that of the turbo exhaust flange.
i wouldnt go too big on the manifold pipe dia as keeping the gasses speed high in the manifold will help the turbo spool quicker, but after the turbo you dont want any resistance.
its not like an NA engine when you need to keep gas speed up to pull the gasses through quicker, you just want rid as quick as possible.