the vf35 is a better turbo for spool. where the vf23 is an all rounder
vf35 is fitted to 04 models some wrx/sti and the vf23 is fitted to the early classic sti 97/98
there is nothing to harm you swapping them over if you can do it correctly
VF23
This turbo is considered a great all-around turbo. Like the VF22 it utilizes the largest P20 exhaust housing. This housing is mated with a smaller compressor housing of the of the VF24. This turbo is considered optimal in applications with range from mild to slightly wild. It does not have the same top end power of the VF22, but spools up significantly quicker
The VF35 has identical internals as the VF30 and it uses divided thrust bearings. However, the exhaust housing is a P15 which means this turbo will have fantastic spool characteristics. This turbo is standard on the new WRX Type RA. The VF30 is commonly considered the best bang for the buck turbo in the IHI VF series line. A relatively new model the VF30 features the same exhaust housing as the VF24 but a larger compressor side similar to the VF22. The combination of these two parts results in increased output potential without the lag associated with the VF22. Although it doesn't offer the top end supremacy of the VF22, the VF30 is a great compromise between these unit and the quicker spooling models.
the vf30+ are a stronger turbo overall and are stronger than all below vf30's.
i have had both of these fitted to my car and i can asure you that the vf35 alone is better than the 23. but it depends on what you want. i have the vf45 now. (not saying anymore about that one though
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main question is what are you after?????????