This may help a lil bit, but I am still researching for ya.
S14
Compressor: T-28, 60 trim 60 mm BCI-1 compressor in T-04B housing
Turbine: T-25, 62 trim 53.8mm 0.64 A/R turbine housing.
Center Section: Ball Bearing

S15
Compressor: T-28, 60 trim 60 mm BCI-1 compressor in T-04B housing
Turbine: Inco turbine wheel. Cast divider wall between turbine discharge and wastegate.
Center Section: Ball Bearing
The turbine wheel is also larger on the s15 turbo.
Max efficient psi on the s14 turbo is about 14 psi. Max psi on the s15 turbo is around 18 psi. Max hp on the s14 turbo is around 280 and on the s15 it's 300+