Here's a picture comparing the #6 and #8 turbine sides on the K26. Note that the outer housing is exactly the same size, it's the wheel on the inside that's larger:
And the resultant differences in their dyno-charts (both on APE chips):
There's a little more boost-lag as the K26/8 takes about 750rpms longer to hit its torque-peak, but from 3500rpm on, it has more torque than the K26/6.