is there any real benefit in going up to the ix turbo? I know there's some gains in going from the 9.8 to the 10.5, but what's the benefit to going up from the viii 10.5 to a ix 10.5

 

The benefits would be very minimal since you already have the larger 10.5 Hotside.

You would gain a larger cold side that can flow a bit more air. Probably not worth the swap unless you need to replace your stock turbo already for some reason.

 

Does anyone know if the Evo 8 10.5 housing have a thinner volute? My E9 has a beefy volute.

 

9 turbo has a larger compressor housing, and a slightly larger inducer diameter on the compressor. It'll make about 20 more whp.